GRU has luggage storage and you have two options: boxes/lockers where you buy coin/ticket and store your lugagge and manned storage. In you case, for such long storage period, I would use the manned storage.

Both are managed by Malex do Brasil. Malex is so famous in Brazil (it has storage facilities all over Brazil) that its name is used instead of luggage storage. In Brazilian such luggage storages are called "Guarda Volumes".

Malex has 4 locations in GRU:

Terminal 1 - asa (wing) B - departures (138 boxes/lockers)
Terminal 2 - asa (wing) C - departures (267 boxes/lockers)
Terminal 1 - asa (wing) B - arrivals (manned storage)
Terminal 2 - asa (wing) C - arrivals (manned storage)
24h per day.

In the arrivals, the location is outside the airport, next to the taxis (Guaru Coopt), just next to the kiosk where you buy tickets for the Airport Express bus which runs from GRU to various locations in São Paulo.

Estimate Prices per day/24h:
Small: 8.00 BRL
Middle: 9.00 BRL
Big: 13.00 BRL

They are 100% safe.

Any additonal info just look for the INFRAERO Airport Information kioks at departures or arrivals.

Rgs,