In part of the recent updates Vriska mentioned Gamzee has "plot armor" and that she will probably throw the fridge into the ocean on earth when all is said and done. Being that Lil' Cal is also in the ocean and both he and Gamzee are on Caliborn's earth, could this be how Gamzee got there? just chillin' in a fridge until earth dries up and a cherub lays an egg there?

Yes, that’s most likely how he got on post-scratch Earth unless he finds a way to get out of the fridge.