there's no point per se in arguing about the level of influence gamzee was under, be it substance, mind control or that of the demiurge² because imo what's actually important (and for the sake of attributing him the little agency he had) is how at every single turn gamzee is implied to have been under the impression he was doing the right thing. murder. clownpocalyse. other miscellaneous downright terrible stuff. he's always acting under the belief that it's all for a greater good and he's doing what he's doing because it all will work out in the end, or in the rare occassion he takes active action for a more selfish purpose, it's when he experiences hatred out of feeling wronged, which is rare—gamzee has withstood all the insults and everyone's less than decent demeanor directed towards him for the entirety of his existence and he never truly minded.
moreover, re: intention, he's a prospit dreamer. prospit dreamers want to be heroes, they're optimistic, they want what they think is best for everyone, and gamzee is the epitome of someone who strives so badly for good they don't realize their evil. he genuinely cared about everything that he was aware of in his little world and he continued to care even as they fucked shit up in sgrub.
to strive for good and all you achieve is harm is tragic, but it's also comedic to the outsider. that's the entire point of slapstick comedy, arguably also why we historically have clowns in the first place as a form of entertainment, because there's something innate that causes one to see something so ridiculously tragic and almost exaggerated, that you cannot help but chuckle regardless of the fool in front of you. this requires ignorance on both parts, and it's related to both the concept of ignorance as bliss and the jester's privilege, mixed in and punctuated by gamzee's entire character.
this all ties in well with the concept of being a "clown" (albeit he is one both by alternian and human standards, which are different, mainly talking about the latter)
and, much like the disconnect required to giggle at the clown who frowns with cake batter smeared across their carefully applied facepaint after getting thrown a plate, everything about gamzee relies on a carefully woven ignorance: his position as a villain or antagonist, rather than victim of the universe's narrative, since it's even implied that seeing him as a victim would be "falling for his manipulation" (when truthfully it doesn't make narrative sense, gamzee /is/ a victim of this universe as much as everyone else), the value of his existence depends on the reader and their preconceived bias, because it wants you to gauge life and stab the less favorable one.
of course a hostile universe would turn hostile against the kid who wanted nothing out of playing its doom game but for spending time with his friends, and the narration is the culprit, all because it needed him to perform a role like a dutiful puppet. his pre-sgrub existence was lonely, spent barely in reality and quelling any attempt at undoing that so carefully woven ignorance. at this point in time he doesn't want anything but to remain in that faux bliss. and for better irony and tragedy, since he had nothing to gain from the game as per the aforementioned ignorance, in turn he lost everything: the idylls of sopor slime, his faith, the dogmatic nature of his ignorance, an autonomy he never truly possessed, and moreover his friends, with him being dubiously aware of the situation, as it's safe to assume gamzee is at all times under an altered state of consciousness.
his level of understanding is limited to what he's been told during moments of vulnerability and clouded judgement, he has no information to possibly counter a lie or misconstrued truth, both due to a neglectful upbringing, and being alone in every sense, as the people he considers friends do not return that kindness. and if they did, who's to say their effort wouldn't be futile? but who's to say they shouldn't try?
for as much as Homestuck deals with themes of enlightenment and fighting back against a seemingly predetermined fate, gamzee instead succumbs to it every time, one way or the other.
the conditions were set and any effort to change them will be pointless because of the way he /is/, because he thinks he's doing what needs to be done and cannot divise an alternative, because he's stubborn and he thinks he's right, because he's been told he's right by possibly the worst actors in their verse.
gamzee is tragically comedic and comedically tragic. he's a clown in the way he is a fool: the innocent pawn that turns into a device to facilitate the universe's narrative with his role as a bard of rage—in a world-ending game of interstellar chess he doesn't even reach the end of the board in order to mutate into something else because his purpose was to be consumed at some point during the battle and that's as beautiful as it is depressing in a plot where the theme is breaking free from that board in the first place.
there's no conclusion to be had here. he did horrible things and had a horrible fate. his character is doomed from the very beginning and there's nothing to be done. the universe is woven in ways it leaves some to fall through the cracks and in spite of his rage, gamzee is one of those. the narrative neglects him as much as everyone else. he was never respected and never cared for until it was too late, and not even a fraction of how much he cared. and unfortunately I adore him and will talk about it