He's nursing the worst hangover ever.
The walk home is punishing because as he's replaying what happened at the party, people from it are walking along the sidewalk with him, talking about how bad their hangover is and how awesome the party was when all Gamzee wants to tell them is to shut the fuck up because he needs to sort some things out. What they believed to be the end to a great party was really the beginning to a series of brutal screams and internal conflicts to Gamzee.
All he remembers is getting into a fight with Karkat, actually Karkat getting into a fight with him because he did something to piss him off. That something happened to be sex and Gamzee can't believe it. He doesn't want to believe it. Every time the sheer pain and hurt and hatred on Karkat's face replays through his mind, Gamzee wants to vomit. What makes the entire ordeal worse is that he doesn't doubt that he did, because he was drunk after all and shit happens when you're drunk.
He breaks off from the crowd of people who for some reason decide to go into town for some drink, and to wreak havoc no doubt. He walks down a road that seems to lead to no where and he's sure it doesn't lead anywhere either; he just wants to walk. He's not in the mood to go home anymore. He can't stand to be surrounded by his things now; he doesn't even want to be himself right now. This self loathing is so intense he wishes he could rip his skin off; peel back the layers of selfishness and disappoint to reach a better part of him. But he doubts such a part exists. He wishes there were a way where he could be someone else, someone less insensitive and less high or drunk all the time. He wants to be someone opposite of how Karkat described him because if he could just get Karkat to smile, that'd mean the world to him.
Thinking of how Karkat stormed away from him at the party hurts too. Karkat was so angry with him that he did even get to apologize, but he doubts he would have apologized to Karkat anyways seeing as how drunk he was. He has a blurred memory of him telling Karkat he was the one who needed to calm his shit, and that maybe he should party. He feels disgusting and actually hates himself even more now. Karkat was only looking out for him, he didn't deserve to have someone like Gamzee- the exact person he always took care of- treat him like he was the problem.
A part of Gamzee, the self centered part of him, tells him maybe Karkat is the problem. Maybe he's the reason why Gamzee drinks like he's dying but Gamzee knows that's bullshit. But that part of him keeps talking- keeps blabbing, it keeps blaming Karkat. And it's starting to sound nice to blame Karkat. It sounds like an okay idea because Karkat doesn't have to look after Gamzee. He just does and Gamzee doesn't know why.
Soon all the ideas his mind is coming up with are indisputable and attractive and agreeable. Gamzee becomes angry, but not with himself, but with Karkat. He decides it isn't his fault. It's obviously not Karkat's fault he had drunken sex with someone but it is his fault he's hurt and angry with Gamzee because he became attached to protecting someone who didn't ask for protection.
"FUCK ME!" Gamzee throws both hands over his eyes but he's not crying.
He's terrible, trying to blame Karkat for his own problem. He's a horrible person and a druggie and an alcoholic and fuck-
He wants to punch something but there's nothing to punch. He tells himself nothing's around to be punched because there's nothing that deserves to suffer from his anger with himself except himself. He changes directions.
He's going to go home and do nothing, maybe die because that's all he feels like doing. He feels like if he just died, life would be better again for everyone and Karkat wouldn't have to watch over him like a baby anymore. Karkat wouldn't have to suffer.
Gamzee walks home, hating the way his phone bounces against his thigh as he steps forward towards his empty home.