There is more than one artist with this name:
1) GAM is a Hello! Project sub-unit consisting of members (and close personal friends) Aya Matsuura and Miki Fujimoto. GAM was created to sing the theme song for the movie Sukeban Deka (starring Aya Matsuura). Their first single, "Thanks!", was released on September 13th, 2006. Their second single, "Melodies", was released on October 25th, 2006. Interestingly enough, this group is becoming infamous within H!P fandom for their lesbian fanservice, as seen in the promotional video for "Melodies".
Their third single, entitled LU LU LU, was released on March 21st, 2007. LU LU LU has sold around 12,000 copies less than Melodies, and ranked 10 on Oricon Weekly Chart.
A fourth single, entitled "Atsui Tamashii", was said to be released in August, but no further info was given on the single. Later, it was revealed that the song would be released on the album Petit Best 8.
GAM's first album was released on May 23rd, 2007. Although there were 9 tracks anticipated, the album contains 11 songs. Apart from 3 singles, 7 new songs it also contains a remix version of GAM's first single "Thanks!". Limited edition has additional DVD with two solo versions of Melodies PV (Aya Ver. & Miki Ver.)+ LU LU LU Dance Shot Ver.
GAM stands for Great Aya & Miki, and is prounounced gyamu. GAM is also slang for "attractive legs" in English.
2) GAM is an American Experimental Punk band that formed in 1991 in Savannah, Georgia. In 1995 they released a 2 track EP called Thunder Bomb on Standard Records and in 1997 released the album Phase 8 on their Blast-o-disc label.
Ronny Kersy - guitar
Susan Gilliam - keybaord
Keith Kozel - lead vocals
Kevin F. Rose - bass
Scott Lankford - drums
3) GAM is a krautrock power trio similar to Ash Ra Tempel. They have two albums: GAM 1976 and Eiszeit 1978. Members: Günter Schickert - guitars, Axel Struck - guitars, Michael Leske - drums.
4) GAM- is a Visual kei band.