Obama secretly planned to return GITMO via demands from the Cuban government.

Breitbart:  Speaking before the United Nations General Assembly, Cuban dictator Raúl Castro insisted that, without the transfer of sovereignty of Guantánamo Bay to the communist regime and the censorship of U.S. broadcasts condemning human rights violations, the normalization of relations with the United States cannot occur.

• Prior to the ultra-sensitive talks conducted by the Obama White House to restore normal diplomatic relations, two of Hillary Clinton’s top aides conducted a two-year secret dialogue with Cuban Foreign Ministry officials focused on exchanging imprisoned U.S. citizen Alan Gross for the “Cuban Five” spies who were serving lengthy jail sentences in the United States.
• The secret talks between White House officials and Cuban negotiators close to Raul Castro came to an impasse in June 2014 over the administration’s demand that, in addition to Gross, Cuba release a CIA asset who had passed intelligence to the U.S. in the 1990s that led to the arrest of the Cuban Five spy network. 
• It was during these talks that GITMO strategies were discussed.  Obama has planned to use the "normalization of relations" between the two countries,  since being defeated in his effort to close the prison during 2009 and 2010.   

Much of the above is text found here.    (bullet points)

BTW,  Congress cannot stop this return.  When the treaty was written giving the US "ownership" of the property on which the new prison stands,  it included provisions for its return and gave that responsibility to the Cuban government in agreement with our president.