Stephen Trask, cocreator of ''Hedwig and the Angry Inch,'' wants you to know something: Gina Gershon can really sing. Trask's latest project is acting as music supervisor for the soundtrack album of the upcoming film ''Prey for Rock & Roll,'' which stars Gershon as the leader of Clam Daddy, a struggling female rock band. Gershon proved to be so proficient a frontwoman that she will handle all her own vocals on the album, as well as in the film. ''Gina is a real natural, just fantastic,'' says Trask. ''She very quickly adapted to what the music called for, which was a classic kind of '80s hardcore-punk style. I think she could definitely do other albums and have a career in music.'' Joan Jett and Iggy Pop will appear as themselves in the flick, which will begin shooting in L.A. this July. As for the album, Trask says there is no label deal in place yet, but he's confident that ''someone will pick it up. Gina is a very marketable person.''