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:
- Annoying Younger Sibling: Gabriel to Raphael, Metatron to Gabriel.
- Backstory: For the angels in Supernatural.
- Beware the Nice Ones: This applies to Michael and Gabriel, if you piss them off enough.
- Big Brother Instinct: Gabriel to Castiel, Samael to Gabriel.
- Cain and Abel: They show up as characters and their situation triggered the whole downfall of Samael.
- Elemental Powers: Michael has earth, Samael has water, Raphael has air and Gabriel fire.
- Emotionless Guy: Raphael.
- 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 lower case gods, such as Apollo and might have happened to the tulpa Gabriel befriended.
- "I Know You're in There Somewhere" Fight: Gabriel when battling Lucifer.
- Lilith: Her origins. Minor character here, though.
- Meaningful Name: Lucifer's original name.
- The Only One Allowed To Kill You: The twins have this attitude towards each other. 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.