Gillberg (under his real name Duane Gill and sometimes teaming with Barry Hardy) became a jobber with the WWF in 1991, usually appearing on WWF Superstars of Wrestling and WWF Wrestling Challenge, losing to the likes of The Undertaker, Kamala, High Energy, The Texas Tornado, Sgt. Slaughter, Jim Duggan, The Bushwhackers and The Beverly Brothers. Gill and Hardy competed in a battle royal, becoming the second version of The Executioners, and took part in a 40-man battle royal won by Tatanka. Gill and Hardy then went back to their real names and began competing on Monday Night Raw as well as WWF Superstars against several other tag teams, losing to the likes of The Quebecers and The Steiner Brothers. One night they fought as The Toxic Turtles and won a victory over jobbers, but the gimmick was cut off. The Executioners split in early 1994, and Hardy left the company on April 18. Gill then began competing as an enhancement talent, losing to the likes of Mr. Perfect, 1-2-3 Kid, Razor Ramon, Doink, The British Bulldog, Tatanka and Adam Bomb. He left the company soon after.
In 1998, Gill made his return to the World Wrestling Federation at Survivor Series as Mankind's mystery opponent. Vince McMahon seemingly facilitated Mankind's route to victory in a tournament for the vacant WWF Championship as he appeared to be McMahon's favorite to win. McMahon built up the suspense before the entrance by referring to Gill as a wrestler with an unmatched win/loss record. Although the statement was implied that Mankind's opponent won more than lost in his career, the exact opposite was true, and Gill was squashed by Mankind.
He later joined The J.O.B. Squad with Al Snow, Scorpio and Bob Holly. During this time, Gill became notable for "ending Marc Mero's career" when Mero challenged him to a match, announcing to the crowd that he would retire from wrestling if he could not beat Gill. Gill won the match with some help from the J.O.B. Squad, and Mero left the WWF.
On November 17, 1998, Gill won the Light Heavyweight Championship after defeating Christian on Raw. After successful defenses against Matt Hardy and Marc Mero, Gill was given his most notable gimmick: "Gillberg," a parody of rival promotion World Championship Wrestling's top star Goldberg. The Gillberg character parodied numerous other aspects of Goldberg's character, such as his entrance being accompanied by the pre-recorded sound of a crowd chanting "Gillberg" and stagehands that would hold up sparklers (parodying Goldberg's pyrotechnics) and then spray the entrance way with fire extinguishers. He also had a dotted line "tattoo" on his right arm (parodying Goldberg's tribal tattoo) and would use the catchphrase "Who's First?" in reference not only to Goldberg's catchphrase "Who's Next?", but also to the fact that Gill would lose to each and every one of his opponents.
Gill made his Royal Rumble debut in 1999, but was immediately eliminated by Edge. Gill's only victory as Gillberg came on the February 8, 1999, edition of Raw when he defeated Goldust with help from former J.O.B. Squad member The Blue Meanie, who was feuding with Goldust at the time. He competed for the WWF Championship against Triple H in a losing effort on the August 31, 1999, edition of SmackDown!.
While he still came to the ring with the belt, the Light Heavyweight Championship was all but forgotten as Gill seldom defended the title on television or at house shows. After being off of WWF television for several months, Gill returned on the February 13, 2000 episode of Sunday Night Heat for one final match in order to lose the championship to the debuting Essa Rios. Upon losing the title, Gill's reign ended at 15 months, making him the longest reigning Light Heavyweight Champion in WWF history. Following this loss, Gillberg would again leave the WWF.
When Goldberg came to WWE in 2003, his first feud was against The Rock, who on the April 21, 2003 episode of Raw brought in Gill, once again under his Gillberg gimmick, to mock Goldberg. After beating up The Rock's security guards, who were trying to apprehend him for interrupting a concert "dedicated" to him, Gillberg attacked Goldberg, which prompted Goldberg to begin choking him. The Rock then attacked Goldberg from behind, after which both Gillberg and The Rock quickly ran out of the arena to avoid further conflict.
On December 10, 2007, Gill, now sporting two new tattoos on his left deltoid, returned to WWE television under his Gillberg name and gimmick for the 15th Anniversary of Raw. During the show, he participated in a 15-man battle royal against fourteen other former Raw wrestlers, but was the first man eliminated only a few seconds into the match by every other competitor.
In 2016, Gill returned to WWE, where he made a brief appearance on The Edge and Christian Show. Gillberg made a surprise appearance on the February 13, 2017 episode of Raw, coming to the ring in place of Goldberg before being attacked by Kevin Owens during Chris Jericho's "Festival of Friendship". On the January 18, 2021 episode of Raw, Gillberg returned. John Morrison and The Miz teased that Goldberg would be on their talk show segment The Dirt Sheet, but instead Gillberg came to the ring and had a short segment with a fake Drew McIntyre.