how exactly was gamzee able to get to caliborn's session, and where did he get his godtier suit? How did he survive being shot hundreds of times? i know that the higher end of the hemospectrum is harder to kill but feferi died just from getting hit with a wand attack, didn't she?

Gamzee was able to get into Caliborn’s session because he stayed on post scratch Earth and waited until the cherub egg was laid there and then took care of them. He entered into the Medium along with Caliborn. Gamzee assembled his god tier suit himself. He was able to survive being shot many times over because it wasn’t his “time” to die yet. Gamzee knew when he would actually die so up to that point he was sort of invulnerable. Also, he’s a rascally clown and they’re hard to kill.