He's an Eternal. As most probably know, they don't die that easily. And Neil Gaimann recently revealed in his mini, that they have a Celestial-created machine, connected to the Earth, that can "respawn" them, if there is actually some force that could dissipate their psi-cosmically controlled atoms.
I have to admit, that I haven't read Simonsons Avengers, but reading up online about it, that he was brought low in one of his stories, by a giant version of a Lava-man(?!) seems rather stupid.
Many other instances has showed that nothing short of total molecular dissipation can kill them. So Gilgamesh nearly dying from a few burn-wounds, doesn't quite float my boat.
Hmm.. I just started reading more online, and apparently that WASN'T what killed him... He, surprise, surprise, "died" in the Crossing. Trough some sort of... chronal energy-infection?? Apparently Immortus/Kang was involved.
We saw a tiny bit of this with Gil in Simonson Avengers...but I'd love to see Gil be back and be the thinking man's version of Ares...."Well he's here what we did at siege of Troy..." All the warrior experience he's had with none of the Ares rashness.