Life isn't just the opposite of death. For Gilgamesh, who starts off a pretty knuckle-headed king, friendship with Enkidu and the trials he endures seem to breathe some life into his consciousness. No more act first, think later for this king. Sure, the answers he comes up with in The Epic of Gilgamesh may not always be the best, but we do see him struggle with the meaning of existence—especially when he gets so much advice from other characters about what he should be focusing on as he goes through life.
Gilgamesh is 2/3 god because of his superhuman strength and endurance; he is 1/3 human because of his mortality.
When Gilgamesh returns home to Uruk, he still believes that death is inevitable for all humans; the difference is that he has a new understanding of the meaning of life.