GOL is not only gaining passengers from TAM and (of course!) Varig, but is getting people that didn't use to fly. GOL's prices are not that cheap for brazilians, that's why most of them travel huge distances in Brazil by bus; but since GOL entered brazilian market, the amount of people that flies, increased.
It's common to think that until last week, when Vasp stopped flying, GOL was still gaining passengers from VASP. No, GOL got Vasp's passengers when started to fly, a few years ago.
I really think that SOME passengers of Varig and TAM, are migrating to GOL, but it depends on many things. In some routes, Varig and TAM's prices are truly high, and GOL's aren't. But in other routes, this difference is not so big, so there's no reason to leave Varig/TAM and go to GOL. Also, Varig and TAM does have a program to earn miles, which GOL doesn't have; this changes a lot, specially for business workers, who use to fly a lot.
GOL is very young and is going on the right way. Is almost impossible to talk about "airlines in Brazil". Majors airlines in Brazil are completely different one from the other. One is too youg and moder, the other is old (a "bit" broken) but has a very known name and certain status, the other has the philosophy of treating the passenger as a king (well, they try to; or at least, they say it). 3 different airlines, 3 different ways of working, fighting for the same market, in a poor country where MOST of people cannot even buy the ticket (business workers are just a few, for the size of the country/tourism is very important, but does not make an airline survive in BRazil). Who will get the passengers? The one with the cheapest tickets. No matter what you do in Brazil. Put the cheapest tickets, and your planes will be full.
BTW, I think it's going a "bit" off the topic (I assume my post is really going off, but that's ok... sorry)