Give it a try once you reach GRU. You never know what is possible in Brasil, but be prepared that you have to take the flight to GIG (in order to protect your return flights form being cancelled). If agents can not/refuse to remove the GRU- GIG leg from your ticket, another option is to buy a cheap ticket GIG-GRU ticket (yes, it will be annoying and more time consuming to fly GRU-GIG-GRU, but at least you will get to Sao Paolo).

I'm sure some agents will be able to understand you in English (suggestion: use Google translate app)