Probably not. If 90% of classical Greek dramatic literature is lost, we can safely assume at the very minimum at least that amount of literature contemporary to Gilgamesh is also lost. There are probably at least 9 other works written around the same time, a little before or after, that would compete with its title as "first." And it is also doubtful something as complex as Gilgamesh arouse out of the blue. There would have been a progression of literature increasing in complexity leading up to it, that is lost.
Thus Gilgamesh is the earliest high literature still in existence. But there definitely had to be earlier high literature, and no judgements can be made on whether that earlier lost contemporary high literature was any better or any worse than Gilgamesh.