Eh... I think Gamzee is "more then mind control" trope. Like, LE pretty much is this opressive narrative force of which Gamzee came to be aware of and his detachment from his friends (who sort of disliked him entire time) and growing sense of despair (and his faith that LE indireclty nurtued thru Alternia culture) caused him to pass mental point of no return when he looked in Cal's eyes and was infodumped by evil creepy doll on the matter of his fate (so only a bit of mind control)

Also, it should also be taken in consideration that Gamzee is Bard of Rage and thus is sort of “vulnerable” to what’s behind “the curtain” (if we take “Rage is a power of plot holes” theory as true) at least until he manages to overcome it (he never really does, despite actively exploiting Rage in his hijinks). So crisis of “we are characters in a story and thus nothing matters” hits him hard. To think about it, he is like… Monika from Doki Doki?