The company that used to be known as GMAC is now Ally Financial.

Ally put the GMAC ResCap subsidiary (the one that included GMAC Mortgage) into bankruptcy in 2012, because of substantial losses; they wanted to get out of the mortgage business and focus on their highly profitable automotive financing business (General Motors Acceptance Company is where GMAC got its name from originally).

This doesn't mean that a GMAC Mortgage LLC doesn't exist, but I'd be wary of dealing with them: if it's "really" GMAC, then they're in financial difficulty; if not, they're deliberately using a misleading name.