Thanks for clarifying, qfrodo. Indeed, the flight I had in mind was JJ3510, which, if I understood correctly, seems to be the only GRU-GIG flight that doesn't clear customs & immigration upon arrival in GIG. See the reply from April 10, 2009, 2:42 AM on this forum.

My relatives live in Jundiaí, so I am counting on a two-hour ride each way, including a buffer for traffic jams. I am also calculating one hour for clearing customs & immigration, and I am counting on being back at the airport one hour in advance, which would yield 5-6 hours of "free" time, assuming no significant flight delays. Does this plan sound reasonable, or should I allocate even more time to clear customs and to get to and from Jundiaí?