Bob looked haggard but was feeling fabulous. Chewing gum at a manic clip, circling the labyrinthine halls of the West Side Club on a recent Sunday afternoon, he had been awake since Friday, thanks to a glassine pouch of crystalline powder he had tucked beneath the mattress of a room he rented in this Chelsea bathhouse. The powder, known as methamphetamine, or crystal meth, had helped Bob conquer a half-dozen sex partners during a hour binge. Like many of the men cruising the two-level club lined with closet-size cubicles, Bob, a year-old advertising copywriter, was ''tweaking,'' high on a wildly addictive stimulant that has been sweeping through Manhattan's gay ghettos. Asked about condoms and the niceties of safe sex, Bob shrugged.