GP has it's busy times, to be sure. If you know whom you'd like to see, I'd advise booking an appointment; if you'd like to see the line up, I suggest checking their schedule (VIA telephone or LL) and coming at the earliest possible time when the most girls are working, which is typically 2pm (I think?). I would not recommend strolling in at 7pm with no appointment, especially if there's a particular girl you'd like to see.
Also, Andrea hasn't been there for awhile; she periodically takes breaks away from the industry. In the past I've called in and got a date when she'd be returning.