the Epic of Gilgamesh isn’t really older than Genesis - see here:
Even if it was - what does it prove? Wisdom, ability to write down considerations and interlocking them with a vage consideration of „a god" has been given to all God-created humans - TO ALL - even to those who refuse to recognize God. All - in opposition of our today’s atheists, saw in nature’s harmony and it’s countless unbelievable wonders, which convinced out ancestors; GOD IS ALIVE! and there must be a or many gods.
To but one of His peoples God revealed Himself in person - to the Jews, Gods selected and beloved People. (As to the question why didn’t God speak to others as well; God let those out, who didn’t want to hear).
When precisely the Jews wrote down what ages ago had been passed on just by the by God protected word of mouth, we don’t really know. Many say around 2,000 or 1,800 bC, other „prove“ it was in the Exile of Babylon 6th century bC.
It’s very obvious, that all cultures of the time or early cultures, some way or another got in touch with the Jewish oral heritage, and also by the into every single human being God-given anticipation of God’s reality - led by heavens, had a basic knowledge of God.
To imagine it was vice versa, pagan early peoples on earth wrote their epics and the Tanakh or Bible then copied it as both sound similar, is plain ridiculous and clear attempt to deny God one way or another! We might well also say; - a good job of hell.
However - Genesis was known long before Abraham, for Noah and later Abraham knew the one and only God as we know God today; or rather as we know today a lot deeper and better, more precise, because God Himself in flesh among us - Jesus Christ, who is God in God, Light from Light, True God from True God, revealed us Got and even the Holy Trinity - as far as we are able to comprehend in our limited breadth of mind.
So strongly limited, because we ourselves keep damaging the formerly open mind in our childhood, by thoughtless intellectual lateral thinking. Intellectual considerations we are tempted to overload our open mind we had and Christ demanded back in Mt 18,3: unless you be converted, and become as little children, you shall not enter into the kingdom of heaven.
Now, this finding back to the open-mindedness of our childhood, is not the opposite of acting as atheists say by „stop believing - start thinking“. No! It far more so is a forgotten, additive divine thinking. A way of thinking that exceeds any by worldly thoughts limited worldly reflections in unmeasurable widths! Hence, believing Christians think much more, than any intellectual atheist would ever be able to think in his very limited ability to put one and one together. Hence the word „stop believing - start thinking“ is the most ridiculous challenge in the world.
To put one and one together means - apart from belief, to be in the state to see countless proves of God’s being. Even a single blade of grass no human ever could make nor ever will be able to make is one out of millions of God-proves creation provides daily.
Earliest and most deserted pagan tribes even saw this, when they adored God in the nature, the sun, the moon, a tree or whatever.
But ok - as God said to the Prophets and to His disciples:
he who wants to hear might hear - he who doesn’t might let it.
All my life I preached Belief, but yes, sometimes I simply gave up, consoling myself with tis sentence, or the one Jesus said to His disciples in Mark 6,11: And whosoever shall not receive you, nor hear you; going forth from thence, shake off the dust from your feet for a testimony to them.