-his general decency (he IS the Gallant after all)
-Olenna's distrust for me
-being nice to Sansa and Tyrion
Garlan being involved is based on:
-Garlan already being proven to not be above treachery
-he is not actually the Gallant, that was just a name given to him by Willas
-more about not being so correct and gallant in that he greedly snapped up Brightwater Keep before Randyll Tarly
-reaction to Joffrey's poisoning
Due to being so cynical, I feel that the first category has more "touchy feely" reasons, while the second has more realpolitik. Which means I am inclined to go with the second, and to forever cross out Garlan as "the gallant" in my mind.
Count me a hopeless romantic, but while I entirely agree that Garlan was in on Joffrey's murder, and that he was nice to Tyrion and Sansa out of policy, I think he can still keep that "Gallant".
After all Joffrey had to die, I think we all would agree, and if Tyrion ended up having to take the rap, well you can't make an omelette without breaking eggs, and Tyrion is no innocent but a full player of the Game of Thrones, and a Lannister. While Sansa brought her unfortunate fate on herself, despite the Tyrell attempt to save her, by blabbing of their plans for her.
While being nice to people can make them drop their guard, it also makes them happier and generally makes the world a better place. There is nothing wrong with being polite, paying compliments, offering consolation, and giving credit where it is due; even when talking to a pawn, opponent or potential enemy.
So, given Garlan seems to be someone who plays the Game of Thrones in a distinctly courteous way, I am happy for him to retain that "Gallant".