Gem has replaced rhythm guitarist Paul "Bonehead" Arthurs, who quit after finishing work on the group's fourth album, Standing On The Shoulder Of Giants. The band have been working on a video for the first single, "Go Let It Out", due for release on 7 February.
Oasis have been pals with Gem since Heavy Stereo - who shared the same record label - opened for them at their show at Cardiff International Arena in March 1996. They also played at the Oasis open-air gig at Loch Lomond later that year.
Gem has been an Oasis fan since the early days. In a 1995 interview he said: "Oasis are an absolute flagship. When they got to number one it was brilliant... You felt proud waiting at the school bus stop."
Oasis still need a bass player to take over from Paul "Guigsy" McGuigan, who left shortly before Arthurs.