I was reading your list of characters status, and I had an idea on how Gamzee reached Earth C : When the head of Jack English (Jack Noir fused with LE) exploded, the fridge with Gamzee inside (as it was on Dirk's planet, where the explosion occured) was sent towards Skaia. Skaia opened a portal and the fridge was sent to the underwater Earth. Rust probably appeared on the locks, and he was able to hide on Earth C. Gamzee (and the fridge, probably) has plot-armor, so he'll survive that.

You forget the head’s explosion made a Black Hole. He wasn’t sent away. He was sucked in. Gamzee, the Crowbar, even Yaldabaoth, are all items that are in Caliborn’s Land, on the future Earth- It’s obvious the Black Hole was a worm hole connecting the end of Lord English (Lord Jack’s version) and the start of his journey (Caliborn), and it makes sense! after all, if the items Caliborn gets come from the defeat of an iteration of Lord English? Then that ensures to close the Time Loop, Lord English has to exist, and become enough of a threat that people would oppose an iteration of it and defeat it, thus sending his Caretaker and the Weapon he used to achieve immortality to his realm.

And that’s the scary thing about Lord English. Even his Defeat is part of the plan. Because he was defeated, he could begin his journey, and become Lord English in the first place. You cannot hope to stop the Demon.

He’s already here.