How else would his trousers stay on once his other garments had been pulled off? But the Hulk's underpants isn't just any elastic material; they're custom-made by one of the Fantastic Four (Reed Richards). ¹⁾

Gamma radiation, unlike vibranium, adamantium, and other elements of superhero technology, is a very real phenomenon. It can originate from space and even mutate and modify DNA, but in the actual world, this generally results in horrifying harm rather than extraordinary power. ²⁾