Gilgamesh can be portrayed as a powerful and haughty figure. He could walk around the world of Danmachi with an air of superiority, looking down on the 'lesser' adventurers. His flamboyant style and his love for grandeur can be emphasized, like always wearing the most elaborate and expensive armor.
The graphic novel presents Gilgamesh as a complex character. He is initially a powerful but somewhat tyrannical king. Then, through his friendship with Enkidu, he begins to change and shows more empathy.
He would likely be portrayed as arrogant. Given his status as a powerful and ancient figure in his original lore, in DxD fanfiction he would probably look down on most of the characters. He would act as if he is above the petty squabbles of the DxD world.
He could be integrated as a powerful enemy at first. He might target the academies because he sees them as places that are trying to control and limit power, which goes against his own philosophy. However, as the story progresses, he could have a change of heart after seeing the selfless acts of the RWBY characters. He could then use his powers to help them against a common enemy like Salem.
In the story, Gilgamesh's age isn't precisely defined. He's presented more as a mythical and timeless hero rather than having a specific age attached to him.
The story of Gilgamesh is extremely old. It was written around 2100 BC and has endured through the ages. This makes it a significant piece of early human storytelling and cultural heritage.
In some fanfictions, Gilgamesh's powers are magnified to an extreme. He can control not just the Gate of Babylon but also manipulate time and space more freely. For example, he can create time - loops to his advantage in battles.