When the Hulk is damaged, his cells regenerate at a fast rate. What happens if his cells mutate into something, or if his DNA gets rewritten by some sort of device? Do his cells prevent mutation?
His immunity states that his cells regenerate at a fast rate.
Regenerate: To form, construct, or create anew.
Say his cells used to be X, but now something made them mutate to Y.
His cells would regenerate and reform themselves into what they originally where, X. They would leave behind the Y and create themselves new X again.
So, I'd say that he would still be immune to mutation. That Gamma radiation is some tuff stuff!
Because even though his DNA would be changed in those cells some others might have the x DNA and even if all the cells were mutated to the y DNA the RNA would still be unchanged and the y DNA could be repaired with the RNA to make it x DNA again. Also gamma radiation changed Bruce Banner's DNA which caused him to become the hulk so since gamma radiation no longer changes his DNA I'm guessing his DNA is pretty stable and would be difficult to mutate.