When he was close to dying Gabriel wished to be born in the world of Tensei shitara slime with all the powers of the Jujutsu universe and when a voice appeared in his head stating that he was copying those powers, Gabriel began to make wishes quickly.... Well for him that would be the best thing that could happen
I may end up making a bit of a mistake about the Tensura universe, after all, it's been months since I've seen anything related to that universe.
Jujutsu powers will have the same cosmology as the Tensei shitara slime universe
He will have the great Sage, but he will not be able to evolve into Raphael until rimuru evolves first
He will also have a shopping store where he can buy things that exist in five different universes.