''Gilbert Gottfried,'' says Paul Provenza, director of The Aristocrats, the notoriously raunchy comedy documentary that killed at Sundance, ''is one of those performers who is just he's like Miles Davis!'' Gilbert Gottfried? It's not as crazy as it sounds, judging from the cheese-grater-voiced comic's hilarious and climactic appearance in this doc (featuring more than 100 comedians), which dissects ''The Aristocrats,'' a warhorse of a joke that ''kind of functions as a secret handshake'' in the comedy world, according to Provenza. The movie, which will be unrated, is uproariously foulmouthed. ''At no point,'' reports Provenza, ''was there any boundary that could be crossed.''