There's no exit control system in Atlanta, just like all other US airports. YYZ to ATL is 100% domestic. On the return from GIG, you'll go through US immigration and customs. You will then go through regular TSA to get to your flight to YYZ. The flight from ATL to YYZ is considered an international departure, but there's no passport control or anything.

As a side note, brasileros can generally understand some spoken continental portugues, but not 100% Portuguese people have a very hard time understanding spoken cariocese, which is the dialect in Rio. Actually, people from the rest of Brasil have a hard time understanding people from Rio too. So, brush up on your brasilian portuguese before you hit Rio.