At GRU prior to taking the escalators down to Passport Control you can just turn to "TRANSIT"-Airline Reps are usually there to help you; This way you'll be automatically sent to International departures; You may eventually go to the upper floor where Admirals Club is-No Showers, I'm afraid.
You'll have to go through Passport Control and Customs if your luggage is tagged to GRU. Usually it's fast; In a bad day it may take you up to 20-30 minutes. After leaving Customs you'll find Airlines Transit Check-in where you can issue Boarding Passes and leave your luggage.
AA at GRU is at Terminal 2, next to VARIG.
(Brazil's Duty free is interesting and nice prices; You may find them both at Arrivals-after Passport Control-The bigger one, or at Departures)
Enjoy !