Castiel/Gabriel
From Super-wiki
Castiel/Gabriel slash fanfiction pairs the characters Castiel and Gabriel in a romantic or sexual relationship.
Despite the very brief interaction of the two characters in canon, many fans feel that there is a story that needs to be told about them.
Sometimes this pairing is considered incest - also called angelcest - depending on whether angels are seen as siblings or not.
Links
- castielgabriel LJ comm for Castiel/Gabriel fic