Gallagher earned a place in the WWE Cruiserweight Classic on April 29, 2016. During the tournament, he beat Fabian Aichner in his first round match and lost to Akira Tozawa in his second round match.
On the inaugural episode of WWE 205 Live, Gallagher defeated Ariya Daivari. On the 5 December episode of Raw, Gallagher defeated Daivari. On the 27 December episode of 205 Live, Gallagher and Daivari participated in a "Gentleman's Duel", where both men chose a weapon for their opponent and began the match back to back before moving five paces away from each other, but Daivari attacked Gallagher while his back was turned until Gallagher attacked him with a headbutt, causing Daivari to forfeit the duel. Their rivalry concluded in an "I forfeit" match on the 17 January 2017 episode of 205 Live, which Gallagher won. On 29 January, at the Royal Rumble event, Gallagher entered in its eponymous match and was the first competitor eliminated by Mark Henry.
On the 7 February episode of 205 Live, Gallagher defeated T.J. Perkins, Cedric Alexander, Noam Dar, and Mustafa Ali in a fatal five-Way elimination match to become the number one contender for the WWE Cruiserweight Championship. He faced Neville for the title at Fastlane in a losing effort.
Gallagher made a guest appearance on the 26 April episode of NXT, unsuccessfully challenging Tyler Bate for the WWE United Kingdom Championship in the main event. On May 18, Gallagher was announced as one of the 16 men entering the WWE United Kingdom Championship Tournament. He would defeat Drew Gulak in the first round, but lost to eventual winner, Zack Gibson, in the second round.
On the 12 September 2017 episode of 205 Live, Gallagher interfered in a match between The Brian Kendrick and Cedric Alexander. However instead of attacking rival Kendrick with his umbrella, Gallagher turned heel by attacking Alexander, before he shook hands with Kendrick, forming an alliance. To reflect his turn, Gallagher stopped wearing his fluorescent trunks and began wrestling in a full suit instead. He also switched to a more technical, ground-based style and began using the umbrella as a weapon. On the 20 February 2018 edition of 205 Live Gallagher wrestled in a losing effort against Mustafa Ali in the first round of the WWE Cruiserweight Championship tournament. In 2018, Gallagher and Kendrick allied with Drew Gulak.
On October 3 episode of 205 Live, after being defeated by Akira Tozawa, Gallagher and Gulak attacked Kendrick, calling him the "weakest link" in the group. On the October 30 episode of 205 Live, Gallagher was defeated by Kendrick after Tozawa prevented the interference from Gulak. The two sides went on to trade victories over the next few weeks. On the December 19 episode of 205 Live, Gallagher and Gulak were defeated by Kendrick and Tozawa in a Street Fight to end the feud.
Starting in December, they attempted to recruit the debuting Humberto Carrillo to their stable. On the April 9, 2019, episode of 205 Live, after Gulak cost Gallagher his match against fellow stable-mate Carrillo by disqualification, Gallagher headbutted Gulak out of the ring. Carrillo and Gallagher then stopped teaming with Gulak. On the May 21 episode of 205 Live, Gallagher teamed with Carrillo to defeat The Singh Brothers. The following week he was defeated by Carrillo in a singles match.
In the following months, Gallagher would find himself in a losing streak, and after losing to Angel Garza on November 29 episode of 205 Live, he would headbutt General Manager Drake Maverick, thus turning heel again in the process. Gallagher returned on February 28, 2020 episode of 205 Live, attacking Lio Rush, while sporting a beard, shorter hairstyle and new tattoos covering his body. With his new attitude, Gallagher would develop a winning streak on 205 Live, defeating the likes of Tyler Breeze and Oney Lorcan. On April 10, 2020, Gallagher was announced for the Interim Cruiserweight Championship Tournament representing Group B in the tournament. Gallagher lost to El Hijo del Fantasma and Akira Tozawa, but was able to defeat Isaiah "Swerve" Scott to secure one win in the tournament, which, however, was not enough to move forward.
On June 19, 2020, Gallagher was released by WWE, with his last match being a loss to Jake Atlas on that night's taping of 205 Live.