Guantanamo predates the Revolution. Cuba can’t kick the US out without starting a war they won’t win, and the US officially maintains the position that the base is paid for according to a treaty they had with the pre-revolutionary government.
Historically, after the Spanish-American war, America founded the Republic of Cuba as a puppet state after kicking the Spanish out, and then they “agreed” to the treaty (along with other provisions like allowing the US the right to intervene in their government), which had no expiration date. So it’s pretty blatantly a part of Cuba that has been annexed by the American Empire for its own purposes.
The failure that will forever define Bernie’s political legacy will be not turning the energy that was behind him in 2020 into a more permanent movement. He just gave the phone bank and email lists to the Dems and fell in line.