What we can infer about Gamzee now...

He was in his right mind all along, that is, everything he was spouting that seemed like nonsense actually made sense.

Let’s think back to the point at which he flipped out. It was talking to Lil Cal that led him to his murderous rampage, but now we know that part of Gamzee from the future was inside that puppet, and it’s highly likely that future Gamzee told his current self what he needed to do to become his own gods, i.e. the mirthiful messiahs (basically to become part of English). He even said that he’d finally figured out who they (the messiahs) were, and that they were always him, which would make sense if he had spoken with part of his future self. They were always him, well, partly him.

He would also have found out that Equius needed to die too, which would explain his need to kill him, as he would need to prototype his corpse with AR. He didn’t need to kill Nepeta specifically, but she lashed out at him, so he would have needed to dispose of her anyway (that and he decided he wanted to kill everyone anyway, though whether he meant that he’d kill them in the future as English is unclear).

It would also explain why he followed Caliborn around and looked after him despite all the abuse he received at the cherub’s hands, as he would need to stay with him and watch over him until the time came for them to fill the empty husk of Cal. It’s probable that he never actually cared for Caliborn at all.

You could say that Gamzee was completely sane the whole time.

In other words, he was totally sober.