Don't forget to check the wiki:

http://www.flyertalk.com/wiki/index....sportation_FAQ

The bus will be simplest, but may be maddeningly slow. If you're traveling light, taking the subway between Penn Station and GCT isn't THAT bad. Try to board the 1, 2 or 3 train at the front of the train. When you get off you just go up one set of stairs right at the front of the train and you're a couple dozen yards from the Shuttle to GCT. The Shuttle runs very frequently during rush hour. The station will likely be very crowded, however, so if you aren't traveling light, it's probably worth just kicking back with a good book on the bus from EWR to GCT (it's the same bus as the Port Authority bus--it just stops at Port Authority first and continues along 42nd St).