The GLCC has good directions and maps on their site, but the public transit directions don't mention some newer bus lines serving the area. For further public transit info, call Port Authority at 442-2000.
If you've never been to the GLCC, note that the door can be hard to spot: it's directly across Forward Ave. from the BP, between the cinema and the pizza place. It opens into a stairway, and you'll need to ring the bell. The entryway can be a little intimidating, but it's a very nice and welcoming space.