On January 12, 2019 at NXT UK TakeOver: Blackpool, Gunther made his WWE debut under the ring name Walter, by confronting WWE United Kingdom Champion Pete Dunne after Dunne's successful title defense.
The following week Walter confronted Pete Dunne and Joe Coffey in ring. Although he did not say anything, he made his intentions for Dunne’s WWE UK Championship quite clear. On the edition of February 2 of NXT UK, Walter made his in ring debut against Jack Starz. He beat Starz in under four minutes. At NXT TakeOver: New York, Walter defeated Dunne to win the United Kingdom Championship, ending Dunne's record setting reign at 685 days.
On the May 22 episode of NXT UK, Walter won a rematch for the United Kingdom Championship against Dunne, after interference by the European Union (Fabian Aichner and Marcel Barthel), thus establishing himself as a heel and forming a new stable called Imperium. The faction would later be joined by Alexander Wolfe, after he interfered in a match pitting Imperium against British Strong Style (the team of Dunne, Tyler Bate, and Trent Seven). On the June 26 episode of NXT UK, Walter retained his title against Travis Banks. On the July 3 episode of NXT UK, Imperium interfered in Moustache Mountain's title match against Grizzled Young Veterans, therefore allowing Grizzled Young Veterans to retain their titles. After the match, they injured Tyler Bate. This led to match against Bate with Walter's title on the line at NXT UK TakeOver: Cardiff, where Walter retained.
Imperium appeared on the September 18 episode of NXT, where they attacked Denzel Dejournette. Walter cut a promo before the group was confronted by Kushida. Later that night, Imperium got involved in a backstage brawl during the Street Fight main event between Matt Riddle and Killian Dain which ended up at ringside. The following week Aichner, Barthel and Wolfe lost to Kushida and his surprise partners, Breezango (Tyler Breeze and Fandango). On the October 9 episode of NXT, Walter defeated Kushida.
Meanwhile, on the October 24 episode of NXT UK, Walter and the rest of Imperium came to the ring before being interrupted by Gallus (Mark and Joe Coffey and Wolfgang) who proceeded to insult Walter and state that NXT UK was “their kingdom” not Imperium's. Two weeks later, On the November 7 edition of NXT UK, Aichner and Barthel attacked Mark Andrews and Flash Morgan Webster while Mark Coffey and Wolfgang attacked Zack Gibson and James Drake during a tag team match between Andrews, Webster, Gibson and Drake. Wolfe and Joe came to their respective factions, before Walter arrived. The two factions squared off in the ring, with the advantage belonging to Imperium, until Ilja Dragunov, whom Wolfe had been trying to persuade to join Imperium, came out to even the odds for Gallus.
On the November 8 episode of SmackDown in Manchester, England, Imperium made their main roster debut (and Wolfe’s return) and attacked Heavy Machinery, before being chased off by the SmackDown locker room. On the November 11 episode of Raw, Walter and Imperium made their Raw debut to answer Seth Rollins’ open challenge to the “United Kingdoms best“. As Rollins prepared to hit his finishing move the Curb Stomp, the rest of Imperium attacked him causing a disqualification victory for Rollins. Street Profits (Angelo Dawkins and Montez Ford) and Kevin Owens ran down to even the odds leading to an eight man tag team match, which Imperium lost after Rollins pinned Wolfe after the Curb Stomp. On the November 22 edition of SmackDown, Walter was one of many NXT wrestlers who invaded SmackDown while being led by Triple H, Shawn Michaels and Road Dogg. Two days later it was announced on the Survivor Series kickoff by Michaels that Walter would be one of the five members of Team NXT in the five-on-five-on-five elimination tag team match happening later that night. Walter would be the first man eliminated in the match however, after being hit with Drew McIntyre's Claymore Kick; Team NXT would also lose the match.
In the build-up for the NXT UK and NXT co-branded pay-per-view, Worlds Collide, Imperium began feuding with The Undisputed Era (NXT Champion Adam Cole, NXT North American Champion Roderick Strong, and NXT Tag Team Champions Bobby Fish and Kyle O'Reilly), which was further intensified during the closing moments of NXT UK TakeOver: Blackpool II on January 12, 2020, when the group attacked Imperium following Walter's successful title defense against Joe Coffey. During his reign the following week, the WWE United Kingdom Championship was renamed to the NXT United Kingdom Championship and he was presented with a slightly updated belt design, replacing the WWE logo at the center with the NXT UK logo. On the October 29 episode of NXT UK, Walter retained the championship in a match against Ilja Dragunov. On the January 14, 2021 episode of NXT UK, Walter would face A-Kid for his United Kingdom Title and retain the title. On February 19, 2021, Walter became the longest-reigning NXT United Kingdom Champion, breaking Pete Dunne's record of 685 days. On the March 17 episode of NXT, Walter would return to NXT and decimate Tommaso Ciampa. It would later be announced that at NXT TakeOver: Stand and Deliver night one, Walter defended his NXT UK Title against Ciampa and retained. On the March 24 episode of NXT, Walter would defeat Drake Maverick. On April 5, his championship reign surpassed the two-year mark. Two days later at NXT Takeover: Stand and Deliver, Walter retained the title against Ciampa. The next day, on NXT UK Prelude, Walter would successfully defend his title against Rampage Brown. At NXT TakeOver: 36, Walter dropped the title to Ilja Dragunov in a rematch, ending his historic reign at 870 days.
At NXT New Year's Evil on January 4, 2022, Walter made his return to the main NXT brand by teaming up with Imperium stablemates Fabian Aichner and Marcel Barthel to face Riddle and MSK (Nash Carter and Wes Lee) in a six-man tag team match, which they lost. After having his final match on NXT UK on January 13, where he defeated Nathan Frazer, Walter was officially transferred to the NXT brand. On the January 18 episode of NXT 2.0, Walter defeated Roderick Strong in the main event, after which, he announced his new ring name as Gunther. On the March 1 episode of NXT 2.0, Gunther defeated Solo Sikoa after two powerbombs, thus ending Sikoa's undefeated streak. In mid-March, Gunther began feuding with LA Knight when he took offense at the latter getting an NXT Championship match by calling out Dolph Ziggler. The next week, after Gunther defeated Duke Hudson, Knight challenged him to a match at NXT Stand and Deliver, which Gunther won.
On the April 8 edition of SmackDown, Gunther, alongside Imperium stablemate Barthel (now known as Ludwig Kaiser) made their main roster debut. In his debut match, Gunther defeated a local competitor with a powerbomb. On the June 10 edition of SmackDown, Gunther defeated Ricochet to win the Intercontinental Championship, making him the first Austrian to win the title. Gunther retained against Ricochet two weeks later on SmackDown and Shinsuke Nakamura on the August 12 episode of SmackDown. Prior to Gunther's title defense at Clash at the Castle on September 3, Ludwig Kaiser announced the reformation of Imperium by reintroducing Fabian Aichner, now known as Giovanni Vinci. Gunther subsequently defeated Sheamus to retain the title. On the November 4 episode of SmackDown, Gunther successfully defended the Intercontinental Championship against Rey Mysterio in the show's main event. He would follow this up with another successful title defense against Ricochet on the December 16 episode of SmackDown. On the January 13 episode of SmackDown, Gunther successfully defended the title against Braun Strowman. At the Royal Rumble on January 29, Gunther entered at number one and lasted until the final two, being eliminated by winner Cody Rhodes. His performance was the longest in history (excluding the Greatest Royal Rumble), lasting 1:11:25. On the February 17 episode of SmackDown, Gunther successfully defended the title against Madcap Moss. On Night Two of WrestleMania 39 on April 2, Gunther retained the title against Sheamus and Drew McIntyre in a triple threat match.
As part of the 2023 WWE Draft, Gunther was drafted to the Raw brand. At Night of Champions on May 27, Gunther successfully defended the title against Mustafa Ali. On the May 29 edition of Raw, Gunther confronted Matt Riddle, and on the June 19 edition of Raw, Gunther would attack Riddle's ankle after a match. A week later, Riddle would challenge Gunther for the Intercontinental Championship at Money in the Bank, which Gunther accepted after attacking Riddle again. At the event, Gunther was successful in retaining the championship. After the match, Drew McIntyre returned and confronted Gunther, who would then take him down with a Glasgow Kiss followed by a Claymore. At SummerSlam, Gunther defeated Drew McIntyre to retain the championship. On the August 21 episode of Raw, Gunther defended the title against Chad Gable but lost the match by countout, ending his undefeated singles streak on the main roster; however, as championships do not change hands by countout or disqualification unless otherwise stipulated, Gunther remained champion. After defeating Gable and successfully defending the title on the September 4 edition of Raw, Gunther officially became the longest-reigning Intercontinental Champion of all time on September 8, passing the Honky Tonk Man's previous record of 454 days. Gunther went on to retain his title against Tommaso Ciampa on the October 2 episode of Raw and Bronson Reed on the October 16 episode of Raw. On November 25 at Survivor Series WarGames, Gunther successfully defended the Intercontinental Championship against The Miz. Gunther went on to defeat The Miz in a rematch on the December 18 episode of Raw.
On January 27 at Royal Rumble, Gunther entered the Royal Rumble match at number eighteen, eliminating Jey Uso, Kofi Kingston and The Miz before being eliminated by the winner, Cody Rhodes. The following night on Raw, Gunther successfully defended the Intercontinental Championship against Kofi Kingston. Gunther defeated Jey Uso on February 19 episode of Raw to retain his title after interference from Jimmy Uso. On February 20, Gunther's Intercontinental Championship reign set another record for the title, this time for most cumulative days as champion, surpassing Pedro Morales, who held the title for 619 days across two reigns. On night one of WrestleMania XL, Gunther was defeated by Sami Zayn for the Intercontinental Championship, ending his historic reign at 666 days and marking his first pinfall loss on the main roster.
Gunther made his first appearance since WrestleMania XL on the April 22 episode of Raw, where he announced he would partake in the upcoming King of the Ring tournament. On the May 6 episode of Raw, Gunther defeated Sheamus in the first round of the tournament. The next week, he defeated Kofi Kingston in a quarterfinal bout, then defeated Jey Uso in the semifinals on May 20 before defeating Randy Orton in the finals at the King and Queen of the Ring event to win the tournament and earn a World Heavyweight Championship match at SummerSlam. At the event on August 3, Gunther defeated Damian Priest to win the World Heavyweight Championship for the first time in his career, notably after interference from Judgment Day member Finn Balor. On the August 5 episode of Raw, Randy Orton challenged Gunther to a rematch for the World Heavyweight Championship after he claimed that he never lost the final match of the King of the Ring tournament, and it was confirmed for Bash in Berlin on August 31. Gunther would successfully retain his title against Orton at the event by technical submission. After the match, Gunther would shake hands with Orton as a sign of respect.
Afterwards, Gunther would begin a storyline with Sami Zayn, who would begin to pursue his World Heavyweight Championship. However, Gunther would reject Zayn's request of a title match on two different occasions until the September 30 episode of Raw, when he finally granted Zayn a title match for the following week on Raw after Zayn brought up the aftermath of their Intercontinental Championship match at WrestleMania XL. At Bad Blood, after Triple H announced future matches to be held at Crown Jewel, Gunther expressed confidence that he would win. He then insulted Goldberg and his son Gage, who were in the audience. Goldberg tried to attack him, but was held back by security guards. Sami Zayn then came out and attacked Gunther. On the following episode on Raw, Gunther successfully retained his title against Zayn in a WrestleMania rematch via technical submission. At Crown Jewel on November 2, Gunther lost to Cody Rhodes, but shook his hand as a sign of respect after the match.