In Gilgamesh Highschool DxD fanfiction, there might be concepts like Gilgamesh forming alliances or rivalries with the devils, angels, and fallen angels in the Highschool DxD world. His vast treasures from his own story could also play a role, perhaps being used as bargaining chips or powerful weapons in the battles that often occur in the Highschool DxD setting.
The devils who oppose Gilgamesh's presence in the Highschool DxD world could be main antagonists. They might see him as a threat to their power and influence in the school.
Issei would probably be the first to want to fight Thanos. He's always eager to prove himself and protect his friends. He might initially underestimate Thanos due to his own growing power and confidence, but soon realize how powerful Thanos really is. Rias would be more cautious. She would likely try to gather information about Thanos first and then formulate a plan to either fight or negotiate with him.
Issei, being the hot - blooded character he is, would probably be excited to meet someone as powerful as Naruto. He might challenge Naruto to a friendly spar at first, hoping to learn from him and test his own strength. And as he gets to know Naruto better, he could start to see him as a brother - in - arms, especially if they face common enemies together.
Rias would be devastated. Issei was not only her servant but also someone she had deep feelings for. She would likely go into a rage at first, vowing to find and punish whoever was responsible. Akeno would be heartbroken as well. She and Issei had a special bond, and she would be in mourning for a long time. Kiba might feel a sense of guilt, wondering if he could have done more to protect Issei. Asia would be in shock and constantly pray for Issei's soul, hoping for some kind of miracle.
Issei would probably be both scared and excited. He's used to dealing with powerful beings, but the Ghost Rider is on a whole different level. He might initially try to fight him out of bravado, but soon realize he's outmatched. Rias would be more cautious. She would see the Ghost Rider as a potential threat to her peerage and would try to figure out his motives before taking any action.
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 might react with hostility. Dark Naruto has a certain edge to him, and if he sees Rias as a threat or someone he doesn't like, he'll likely oppose her. He could use his combat skills to challenge her directly.
Some characters might be intrigued. For instance, Issei might be curious about Fem Naruto's fixation on Ophis. He could see her as an interesting addition to their world and might try to understand her motives. Rias, on the other hand, might be more cautious. She would likely be concerned about how Fem Naruto's admiration for Ophis could affect the balance of power in their territory.
First, do your research on each of the source materials. Know the power levels, the personalities of the characters in Naruto, Gilgamesh's lore, and the world - building of DxD. Then, come up with a unique concept that can blend these three elements together. For example, you could have a crossover where a Naruto character is transported to the DxD world and meets Gilgamesh there.