GIG-GRU can be booked using OWE tickets -- it is not cabotage. It cannot be booked using point-to-point tickets. The reason it can be booked is that it is part of an international ticket -- and traffic rights exist for that. A bit murky is whether there are any routing restrictions on the sector (I think you must have an international flight entering/leaving brazil immediately before or after it ... as you would in any case).