This section of the Wikipedia article does an excellent job of explaining President Obama's actions to close the detention camp, and how Congress interfered. They've made it impossible for the President to lawfully transfer the detainees to any US state or foreign country. There is realistically no way for the President to release them without being able to transfer them.

https://en.wikipedia.org/wiki/Guant%C3%A1namo_Bay_detention_...