The AMEX GRU lounge is decent:
-There's self-service snacks and non-alchoholic beverages.
-There are 2 free internet enabled computers.
-There's a nice bathroom.
It's located landside, one floor above ticketing.
It's called the centurion lounge, but the name is a misnomer, my AMEX Plat got me in along with 2 guests for free, and AMEX Gold should get you in as well.