Obama still didn't do it. true, he should never have promised to do it, but the Guantanamo Bay detention facility was not created by congress, and therefore did not require congressional approval to close.
ObamaCare, by contrast, is a law passed by congress (albeit without a single Republican vote) and signed into law by the President. Repealing it will also require congressional approval.
The two promises are quite different as they relate to the constitutional scope of presidential authority.