SBGR/GRU :
First thing to remember : it's hot and high

Which means HDA operations.
So in order to avoid the usual "rushed approach" feeling during HDA operations, configure early and adapt your speed (but I guess you already know all of this). It's really easy to get high and fast. Especially since there might be some tailwind on final.
Thunderstorms are frequent and can be pretty intense. Careful with wet runways in this case.
Many many small airfields in the vicinity which can mean busy ATC at times.
Preferred landing is 27L/09R but even if ILS 27L is in use be aware you may be cleared for 27R in the end. ATC is... flimsy at best tongue
Careful with the procedure names : can be a little bit confusing at times. Especially when listening to clearances. So listen carefully to the ATIS and ALL the ATIS ! And then cross-check the charts. Lots of approaches based on NDB so check the margins well.
North of the field, some sectors are higher than transition altitude which means minimum altitudes can be levels.
Careful when you park (watch the signs)
I don't know what kinda plane you're gonna fly but remember that because of altitude, engines should be started individually.
Audio ATIS can be received pretty late.
All I can think of for now. Ah yes, It's Cat B !
Rio is a wee bit more complicated since it's been quite a while...
SBGL/GIG :
Same thing for minimum altitudes north of the field so be careful with minimum Z !
Same thing about tailwind on final and some approaches are quite steep with limited speed so again : configure early. And ATC might report "wind calm" even if you've got tailwind so don't be too trusty when it comes to ATC wind reports.
You might have to catch up with the glide on final because you end up arriving high for rws 10 and 15 (at least was that way when I got there, might have changed though). Which means again, configure early.
Watch out for fog, especially at this time of year.
ATC is the same as previously : flimsy at best

Always anticipate the next frequency just in case, and now where your frequency list is. And watch out for radio failures
If you want to get used to the ATC accent before leaving : have fun listening to liveatc, maybe they're online.