You can try fanfiction websites like FanFiction.net or Archive of Our Own. Just search for 'Highschool DxD Issei x Gabriel' on their search bars, and you should be able to find relevant fanfictions.
One possible plot could be that Naruto is somehow transported to the Highschool DxD universe and meets Gabriel. They might team up to face a common threat that is endangering both the ninja world and the supernatural world of Highschool DxD. Naruto's unique skills as a ninja could be combined with Gabriel's angelic powers in an epic battle against evil forces.
The fanfictions also allow for a deeper exploration of the characters' personalities. Issei's determination and Gabriel's grace can be further developed. Writers might give them new backstories or put them in situations that are not in the original anime. This gives readers a fresh perspective on the characters and their relationship, which is what makes these fanfictions so interesting.
Well, the plot could involve Harry being a new transfer student to the Highschool DxD world and Gabriel being one of the first to befriend him. Maybe they team up to face some of the common threats in that universe, like the devils or fallen angels. They could also explore their own unique powers together and learn from each other.
One possible fanfiction could be about a crossover where the characters from Highschool DxD and Naruto are transported to a world where they meet Gabriel. Maybe they team up to fight a common enemy. For example, the devils from Highschool DxD and the ninjas from Naruto could combine their powers under Gabriel's guidance to defeat an ancient evil that threatens all three of their realms.
Maybe an evil sorcerer who has mastered both dark magic and some forbidden ninja techniques. He plans to use the chaos that would ensue from merging the two worlds to his advantage. He would be against Naruto and Gabriel because they stand in the way of his evil plans. He could start by creating unrest in the Highschool DxD world, pitting different factions against each other, and then try to use Naruto's presence as a scapegoat. Gabriel, being an angel of justice, would naturally oppose him, and Naruto, with his sense of loyalty and protecting others, would join her in the fight.
They could have a mentor - mentee relationship. Gabriel, being more familiar with the Highschool DxD world, could guide Harry. For example, teach him about the different factions like the devils, angels, and fallen angels.
One popular plot idea could be the OC being a new student at the school and catching Gabriel's attention. Maybe the OC has a unique power or ability that Gabriel is intrigued by. Another idea is a story where the OC is in some sort of danger and Gabriel has to protect them. This could lead to a deeper connection between them as they face challenges together.
To write a good OC, start with giving them a distinct personality. For example, if you want them to be a contrast to Gabriel, make them more impulsive or hot - headed. Also, give them a backstory. Maybe they come from a family with a dark secret in the supernatural world. This will add depth to their character.
Well, I know a fanfic where Issei and Gabriel team up against a common enemy. In this story, their relationship blossoms as they face challenges together. It's really interesting to see how their different personalities complement each other in the story.