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.
In Avengers Forever we lerned, that most of the involved heroes in the crossing-saga were Space-phantoms in disguise. So I think,the gilgamesh who "died" was an spacephantom. It was never revealed or mentioned.
And Marrina I saw on the cover of "chaos war: alpha flight" with other "dead" members of Alpha Flight "back from the great beyond"