We used the VIP/Smiles lounge 5 weeks ago, when taking US Airways biz (Envoy) class to CLT. Not great water pressure or warmth, but quite serviceable. Ask to get put on the list as soon as you check in, in case there's a wait. The lounge is down the stairs you'll see a ways to your left as you enter the gate area of Terminal 2. Can't recall if the sign at the top of the stairs says VIP or Smiles (Smiles used to be the Varig ff program). There's a cafe/fast food place just outside which may have a very long line, but just wind your way through that. The lounge has a limited but decent array of snacks and drinks, including alcohol. It can get rather crowded. Since you say Global First rather than Business-First, I gather you're flying to Washington or Chicago rather than Houston.

I think UA flyers may have used two lounges when former CO flights were still using Terminal 1. If I read the signs inside and outside the airport correctly, all UA flights (including former CO flights) now use Terminal 2, where the VIP/Smiles lounge is located. BTW, I think the best place for actual dining, if you need that, is in the Luxor Hotel's restaurant in Terminal 1 (well, at least better than the food court in the same terminal). It is a long walk from Terminal 2, unless you can grab a seat on one of the carts that go back and forth. In the VIP lounge you can find small snacks, and maybe soup.

The Luxor hotel in the airport charges about $200. No day rate, we were told. There is a basic pousada in the basement of the airport. The lounge looks like your best bet.

If XLR26 is talking about Terminal 1, yes, it's a bit of a dump. There were problems, including underdized/slow/malfunctioning elevators in case you have bags you need to take between floors and taking them up an escalator isn't feasible. Extra room taken by an attendant in each one. The airport looks more updated than GRU, but that's not saying much, and I'd hate to be there when crowds descend for the World Cup and Olympics. Hopefully improvements will be completed.