One possible story could be that Shiro, as a powerful female, becomes an equal to Gilgamesh. She might use her unique powers to challenge his authority in the fanfiction world. Maybe she has some sort of magic or combat skills that make her stand out. She could start as an unknown figure but gradually gain recognition and respect from Gilgamesh and other characters in the story.
Grand Legend was a Chinese online literary novel. There was no clear protagonist or the strongest character because the plot and setting of the story were very complicated, involving many different fields and skills.
However, Gilgamesh was the main character in the Legend of Gilgamesh. He was a mythical figure in ancient Indian legends, believed to be the embodiment of gods and heroes. Gilgamesh was extremely powerful and intelligent. He was proficient in music, poetry, and war. He could also manipulate natural elements and magical power. He had experienced many adventures and challenges in the Legend of Gilgamesh, and he had shown great ability and wisdom, so he was considered one of the most powerful characters.
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.
She could be a rival to Gilgamesh. With her power, she might aim to take over his kingdom or prove that she is a better leader. This would create a lot of tension and interesting power dynamics in the fanfiction.
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.