Snowden would not have been sent to Guantanmo because he is an American citizen. The point of Guantanmo is that the US wanted a place to keep people who weren't US citizens and wasn't on US soil.
Not just that. They had all these detainees that they thought were probably terrorists, but they knew they didn't have enough evidence (or at least admissible evidence) to convict them. So, between sending them court knowing a lot of them would be acquitted or letting them go back to do whatever bad stuff they were worried would happen, they just decided to hold them in limbo. They have plenty of evidence to convict Snowden in a civilian court.
