G-SPA The Hotel Gansevoort, 18 Ninth Ave. Opening: February THE PLAYERS: Richie Akiva and Scott Sartiano, the duo behind celeb-strewn Butter in the East Village, are teaming up with China Grill’s Jeffrey Chodorow. THE PLAN: The glitzy subterranean lounge will function as an actual health spa for hotel guests and “select†others during the day and transform into a clandestine den of sin for the rich and fabulous come sundown. Sartiano — the smooth operator who once dated Ashley Olsen — asserts that G-SPA will serve as a “much-needed†new venue for his high-profile buddies. THE SPACE: The 3,000-square-foot venue — bedecked in cool white tiles — features three hydro-pools for the spa’s daytime guests. Nightfall could elicit water sports among expected celebrities, as there are banquettes alongside the tubs; though Sartiano says, “Of course, the pools will be off-limits during lounge hours.†The spa’s treatment rooms will morph into curtained-off, posse-sized nooks, with flat-screen TVs and mini-bars. THE DOOR: Among the toughest in town. Co-owner Sartiano has recruited a doorman “who nobody knows,†in order to keep the usual suspects out. Favoritism is expected to rule. If you are known at Butter, you’ll be welcome at G-SPA. And of course, unless you already are on the proverbial list, it can’t hurt to come with a posse of pretties. THE LIST: You know — “the gang!†Paris Hilton, Nicole Richie, Lindsay Lohan, Mischa Barton, Diddy, Molly Sims, Ashlee Simpson, Taye Diggs, John Stamos, Anne Hathaway and Estonian uber-mannequin Carmen Kass (who had a long on-again, off-again romance with Akiva).