NAME Will Kemp AGE 25 WHERE YOU'VE SEEN HIM So he's the stunner grooving in those loose-fit jeans in Gap's ''For Every Generation'' campaign. COMMERCIAL SUCCESS A professional dancer who trained at London's Royal Ballet School (his credits include 1998's all-male ''Swan Lake'' on Broadway), British-born Kemp had no problem falling into the Gap: ''The Gap has tremendous, clever ads, so when I was approached, I was like, 'Absolutely.''' Besides, he says, ''I thrive on [being] able to do a high-profile job like the Gap campaign, then come back [to London] and hide in the obscurity of a small-scale theater production.'' IMPROV-ABILITY ''I don't think [the director and crew] knew what to tell me, and I didn't know what they wanted,'' says Kemp of the music-driven spots. ''I just got my rocks off and jumped around for a bit.'' NEXT More theater, and his big-screen debut opposite Val Kilmer and Christian Slater in ''Mindhunters,'' due next year.