UA is not permitted to sell flights intra-Brazil. You can only take the GRU-GIG/GIG-GRU flights if it's part of a connection to/from the US. That's why it doesn't come up when you try and book it as a one way. Similarly.. you can't purchase flights on UA from SYD-MEL in Australia.. but they do fly it.
Note flight 861 in the above example IAD-GIG shows a stop in GRU. Your desired flights do indeed show up at United.com but they're quite a bit more expensive than what you can get from Hotwire.
Last edited by SEA1K4EVR; Nov 26, 2012 at 3:32 pm