You see Andrew hussie had a reason of making him like that since his characters are based off Tumblrs old days there where "Juggolos" that hated or didn't know anything about ICP
Idk where this comic fits in timeline-wise, so I'mma just fill that question with my own thoughts and say that this is an alternate timeline where Gamzee managed to break free of English's control somehow and has severe trauma relating anything to ICP and clowns. Like him about to go insane isn't him fighting actual violent urges, but him about to breakdown over the fear that he almost succumbed to them. Edit: Okay, so I completely missed the scars on his face, so I know when this happens now, but I wanna have some fun and say Nepeta scratching the hell outta his face snapped him out of it. Why? I like Nepeta. She needs this win.