I'm similarly situated to the original poster. AA long-haul into GRU, premium cabin. Possible G3 onward flight (domestic within Brazil). I'd have 65 minutes (based on flight schedules). Separate tickets. No checked bags.

What should I expect for passport & customs at GRU for a US citizen at peak time on a weekday morning?

Could buy a refundable ticket on G3. Then it hurts less if I miss the connection.

Would certainly want to download Gol's app to do check-in that way, or print boarding pass before departure from US the night before. Any reason to think I wouldn't be able to do this?

Coming back, I'd have 85 minutes. In case of misconnect, I'd be on a flexible AA ticket so could change to a later flight with assistance of AA staff and pending inventory (which doesn't look like a problem).

Definitely not ideal. Odds seem ever so slightly better than the original poster, 65 minutes rather than 50, which is a big difference. Makes me pretty uncomfortable though.