My thoughts are I don't think so. I have to ask, whose theory? I never heard of that theory! Unless the writer, Brandon Jerwa, or Andrew Modeen who is listed as Continuity Research, said "Oh yes, I decided that was Gilgamesh!" it really doesn't matter. It can't make sense just because of the time and place of the character's death. All we know about the Highlander Gilgamesh is that Amelatu, the Akkadian, created the Watchers after learning about him, over 4,000 years ago. As it's 2021 now, the Kurgan took that Immortal's head 2,634 years ago. I think. If it really was 4,000 years from 1998, then Ameletu's discovery was in 2002 B.C.E. The Kurgan took this man's head 1,387 years later. In the comic, the Kurgan was supposed to be 25 and "somewhere in the Mediterranean" in 880 B.C.E. He was born in the Russian Steppes and left his second foster mother when he was 12. Of course, all of these are huge regions and the writers are being intentionally vague, but in theory he had traveled over a thousand miles in that 13 years.