How did Sumerians view the inevitability of death in the Epic of Gilgamesh?
The first part of the answer is included in your question: the Sumerians saw death as inevitable. No one, not even Gilgamesh, can bring a loved one back from the dead.
As a result, it becomes (in the poem, and perhaps in the world) one of the things that defines humanity. We are by definition mortal. We die, and the gods and the world go on.
Also as a result, death is one of the things that humanizes people. Gilgamesh is impressive before Enkidu dies, but he is also shallow and arrogant. After his loss, he is more likable.