This assumes outright brainwashing. I like Liu's idea in AXM that Deathbit was always a part of him ie a manifestation of his darker side born from his lack of self worth and feelings of rejection. In that vein then Rogues rejection of him provides the opening for his darker side to emerge and the resolution comes in two parts.
First he sees footage or memorials of people remembering him and his sacrifice from all across the globe including Kurt's eulogy.
Second he somehow relives the gala but this time maybe he stays and hears Rogue tell Mags that he was right and some things are deeper than skin. With that he remembers Xavier's last words to him ie “Gambit, how often must a scoundrel prove himself a hero, before he believes it himself?” And with that the hold Deathbit has over him is broken.