…but did Odin seriously not recognise his own son?
Either Gabriel is seriously talented, even more so that the show conveyed, or at least one of the other gods in that room was (or is) playing a longer and more intricate game then the show let on.
Do not confuse Marvel comics with Norse Mythology.
Loki in mythology is not Odin’s son.
Loki was Odin’s “brother” by blood contract. A very long time ago Loki and Odin made a pact, where Odin agreed for Loki to be his brother, granting him a place among the gods by his side. Loki’s origins are unknown in lore, even for the other gods, but it is said that he is the son of giants (no, not like the frost giant from the comic books, but the actual giants in mythology, although they are similar). The other gods believed his giant blood to be the reason he has monster children, like Fenrir (the big wolf), Hel (goddess of death and the underworld) or Jormungand (the world snake).
In lore it is Loki who gives the gods the most powerful weapons. Like Thor’s hammer and other relics. Almost all the famous swords and jewels are given to them by Loki… maybe that was what he promised Odin at the beginning when they met? Maybe that was the deal?
So if you think that maybe Gabriel showed up one day and offered something to Odin, who in return declared him his brother among the gods. It fits.
I do not know whether Gabriel created the persona/identity of “Loki”, or whether there was an actual Loki before him as it is not stated in Supernatural canon.
But not even the original Loki in mythology was anyway truly related to any of the gods. Odin granted him the status of a god in that pantheon.
It’s my personal headcanon that, after the whole ‘Baldur Incident’, Loki really needed to get the hell out of dodge - and it just so happened, Gabriel did as well. And since Loki is a shapeshifter, none of the Norse gods would have batted an eye if Gabriel didn’t even bother to try and impersonate Loki.
And because the Loki!Fangirl in me can’t resist sharing knowledge: Most of the Norse Gods were, at least, half Giant. While Loki is (most likely) pure Giant, not all of his children ended up Monsters: He had three wives that we know about: Angrboda - the mother (or at least other parent) of the famous monster children, Sigyn - The mother of the twins Vali and Nari (Both of which were perfectly Non-Monsterous), and the least known first wife: Glut - A Giant Hearth Goddess who bore Loki two Daughters (also Hearth Goddess), Einmyria and Eisa. And lets not forget the Horse incident.
Also the times Loki would decide to hide on Earth for a bit as a milkmaid and have shit loads of kids then as well.
So, in short: If Gabe took over to Loki, he’d going to have to remember a lot of birthdays. If Gabe was Loki all along: He didn’t waste any time getting catching up on the whole sex thing, did he? XD