So if Gamzee isn't actually God Tier, how is he so resilient that he's able to survive being shot at multiple times?

Plot Armor and Paradox Bullshit.

He couldn’t die because if he did then he wouldn’t be able to become part of Lord English. And if he didn’t become part of LE then Gamzee’s religion wouldn’t exist. As well as a few other things but I don’t want to make this more complicated than it already is.