Well, it could be. If Percy is in a Gilgamesh fanfiction, it means that the story is likely using the Gilgamesh mythology as a base. Percy could be a new character created within that framework, perhaps having adventures that are related to the themes and stories found in the Gilgamesh epic. It might also mean that Percy has some connection to the original Gilgamesh character, like being a descendant or a reincarnation in the fanfiction world.
One connection could be power - related. Gilgamesh is often depicted as extremely powerful in his own mythology. Percy in the fanfiction might also possess great power, and perhaps his power is modeled after Gilgamesh's in some way, like having a similar weapon or ability.
One idea could be that Percy and his friends encounter Gilgamesh during a quest in a strange new realm. Gilgamesh could be a powerful ally or a mysterious foe. Maybe they have to work together to defeat a common enemy that is threatening both the world of Percy Jackson and the ancient lands of Gilgamesh.
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.
Well, another place to look could be Tumblr. Sometimes fans post their fanfictions there. Also, Reddit might have some communities dedicated to Percy Jackson and Gilgamesh fanfiction. Check out relevant subreddits and ask the community if they know of any good ones.
Well, often in Percy Jackson fanfic, Gilgamesh is presented as a character from a different mythology but still manages to fit into the world of demigods. Sometimes he's seen as a potential ally. For example, his great strength and battle experience could be useful against common enemies. Other times, he could be a bit of an enigma. His motives might not be clear at first. He might be interested in the power of the demigods or the magical items in their world. His appearance in fanfic can add an interesting layer of depth and mystery to the story.
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 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.
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.