They almost certainly can't send him to Gitmo, if he's been extradited on formal charges. That puts him in the hands of the regular legal system. What makes Gitmo a special case is that it's an end-run around both American and international law... prisoners there aren't in the American legal system and subject to awkward rights like habeus corpus and fair trial, but likewise aren't prisoners of war and subject to the Geneva Convention (which calls for freeing enemy soldiers as quickly as possible unless they are being charged with war crimes for an international tribunal, which is highly unusual).
This is why Gitmo is a shame on our national honor, but it also means Assange won't be going there.