Deeply, but though we can make distinction between a modern best seller and Brian's grammatically incorrect graffiti, "Romanes eunt domus,"[1], in essence they are both literature, even if the latter is fictional and framed within the larger contemporary story, because Brian wrote a message representative of some language.
Literature is defined by writing, and writing is defined by symbols representing language. Always given less emphasis than the ancient cave art found in caves all over the world are the abstract symbols that very often accompany the graphic depictions of animals. For all we know some of those symbols may mean, "this tastes good," and regardless of being unable to decipher them, they could still be the oldest extant literature[2] so long as they could represent language and tell a story, no matter how primitive a story.
Before the Egyptian hieroglyphs and Mayan pictographs were deciphered, there was no question that they had specific meaning behind them. Though in the strictest sense, pictography is not an alphabet, it still conveys specific messages in some once spoken language. In my opinion, if there are any abstract symbols that represent even the most primitive message, if it's clear there is even some vague attempt at communication, then it will still broadly fall under the category of literature, regardless of ever being deciphered.
While ultimately I agree that the epic of Gilgamesh is the earliest extant high, advanced, or complex literature, I think whether a written story still exists or not is an arbitrary distinction. The fact that Gilgamesh exists in the literary form that it does necessarily means earlier examples of high literature once existed but are unknown and likely lost. Between Aeschylus, Sophocles and Euripides there were over 300 plays written of which 33 survive. That the missing plays no longer exist should not diminish the fact that these were, in fact, written and were undoubtedly literature.
[1] https://en.wikipedia.org/wiki/Romani_ite_domum
[2] https://www.openculture.com/2019/03/40000-year-old-symbols-f... note: Though von Petzinger herself clarifies in her TED Talk that the common ancient symbols are not writing, I think she's hedging for some reason (probably professional caution) contradicting what her own research shows and statements she makes immediately previously.