As Wikipedia explains, there is in fact an agreement, which predates the current Cuban regime. Cuba has protested it many times with plausible arguments but since the agreement is already in place, they don't have much leverage. Obviously, Cuba cannot confront the US militarily, especially now that they don't have the Soviet Union to back them anymore, so they would need to offer something to get the US to leave Guantanamo, not the other way around.
Furthermore, it's not clear to me that Cuba strongly wants the embargo lifted. It's already able to trade with most of the rest of the world anyway and normalization of the relationship with the US could bring many risks for the current Cuban regime, including losing its main excuse for everything that goes wrong. It's very difficult for the US to renounce this policy for internal political reasons but the embargo is widely considered to be ineffective or even self-defeating.