The number of Casdreel AUs I have kicking around in my head is alarming.
College AU where Gadreel and Cas meet at a frat party. Gadreel is drawn to Cas, who's awkwardly standing off to the side and clutching a drink he hasn't taken a single sip of all night (not that Gadreel's been watching him.) Gadreel goes to talk to him and learns his story, and they become cutie friends.
...Alternatively, an AU where Cas and Gadreel wake up from a crazy, drunken one night stand. Their respective friends have never seen them party so hard and be so crazy for another person. Cue "it was just a one night stand, it doesn't mean anything" rivaling actual feelings and stuff. (This was inspired from hearing Stay With Me too many times on the radio.)
Coffeeshop AU where law student!Gadreel is a regular customer at the shop Cas works at, and there's enough fluffiness and cuteness to rot your teeth.
And the College AU I really wanna write where Cas brings Gadreel home for Christmas because Gadreel has no where else to go. Cue holiday family shenanigans with Cas' four older siblings as Cas and Gadreel ~realize their feelings~; also, mild Michifer on the side. ("Cas, I think Michael and Lucifer are....involved." "We don't talk about it.")
There's more, but these are the most fleshed out ones I have so far. Dumb babies and their cuteness.