A History of Heaven and Its Angels, as Understood by the Archangel Gabriel is a Supernatural fanfic by Fantismal on Archive of Our Own found here. The fanfic details the background of the angels, starting with a young earth and ending with Gabriel leaving Heaven.
It has a sequel and some other spinoffs as well, if you click on the author's profile on Archive of Our Own.
A History of Heaven and Its Angels, as Understood by the Archangel Gabriel provides examples of:
- Cain and Abel: They show up as characters; their situation triggered the downfall of Samael.
- Elemental Powers: Michael has earth, Samael has water, Raphael has air and Gabriel fire.
- The Evils of Free Will: A running theme for this fanfic and its spinoffs.
- God: As Himself.
- Gods Need Prayer Badly: This happens to the lowercase "g" gods, such as Apollo, and might have happened to the tulpa Gabriel befriended.
- The Only One Allowed to Defeat You: The twins have this attitude towards each other, as each feels he's the only one who should get to kill the other in the end. Raphael tries to do so at the end, to prevent Gabriel from leaving.
- Our Demons Are Different: As per the series, they were originally human and were tortured into becoming demons.
- Our Souls Are Different: The angels don't have souls, per se. Not like humans.
- Satan: This is both before his Fall and after when he is imprisoned in the Cage.
- Thou Shalt Not Kill: Gabriel is reluctant to kill another angel for awhile.
- Tulpa: He becomes important towards the end.