Trouble is, how do you make a Servant that's "basically Gilgamesh but also not Gilgamesh"? She works in Prisma because she's the only (adult) Gilgamesh running around, but it'd be hard to give her a unique identity when OG Gilgamesh is still running around, either in skills or in personality (since without Gil's card she basically has no personality and her only ability is the Einsworth magecraft).