Are the seats available for revenue purchase? If so, they can be yours for EasyPass. I suspect the flights are now sold out and the co.com system isn't quite real time, so it was still displaying them.

Other options are routing thru IAH or EWR (-IAH/EWR-GRU) or seeing if availability exists on the same flight with GIG as a final destination (EWR-GRU-GIG), then you can hold that then cancel the GRU-GIG segment later.