Okay now that I've spread my propaganda, I can properly answer this ask. I'd normally give a few "pre-get-together" scenarios but I literally have everything I could ever want for that from canon valorPUNK, so instead you get
GAVRUSS DATING HEADCANONS 🎉
Gavin is a good and polite teacher's pet until Russel's god-awful family (get it?) comes around. Then he goes small dog mode and starts barking at Zeus for being a terrible father, for putting his own SON in danger, how dare he— and FREYA, agreeing to give Russel to human parents to avoid a god-war that was her fault in the beginning, but then hanging around him all the time like a creep anyway? He's not your son anymore! Leave him alone! And Apollo and Athena, if you threaten Russel I will FIGHT YOU HERE AND NOW, I DON'T CARE THAT YOU'RE GODS—
Russel constantly forgets to charge his phone because of the nonstop chaos of his life. The first time he asked Gavin for a charger he had to build one from his bag; after the second time, Gavin just started carrying around extra chargers and portable battery packs.
Russel is the most awkward boy forever and nothing will ever change that. He'll spend 15 minutes fumbling to ask Gavin if he wants to go out to eat. "Russel are you asking me out?" "Um. Yes. If you have time though I don't want to pressure you if you're busy—" "Russel we're literally already dating." "BUT I DON'T WANNA PRESSURE IF YOU'RE BUSY ;~;"
(This is NOT due to any sort of special being flustered around his bf. Russel is literally just always like this and I think it's important nothing ever changes that.)
One reason he's so polite and meek is because Russel's seen Zeus and how Zeus's actions affect people, and he does NOT want to become that. Gavin loves how polite Russel is but is also lowkey frustrated because it forces him to admit when he wants affection and ASK for it which is VERY EMBARRASSING, will his stupid bf take the hint and PIN HIM TO THE WALL ALREADY—
Gavin is the kind of person to show affection by memorizing every detail about a person and remembering everything they like and dislike. Russel is the kind of person who's not good at showing affection until the chips are down and suddenly he's throwing his life on the line to save the people he cares about.
Gavin knows NOTHING about sports but loves hearing Russel talk about something that he likes for once in his life, so he'll purposefully ask questions about what sports program Russel is watching, when did he get that jersey, why did that guy get penalized, etc. to try and spur him into an infodump. Nerd x Jock in the healthiest way possible.
Gavin suffers nightmares pretty frequently. Russel is used to being woken up in the middle of the night by screaming (thanks to being roommates with Vika) and calms him down and offers Gavin to sleep with him. Congratulations, Gavin Grey, you have trauma! Your prize is a multi-pantheon god as a teddy bear.
If I had a penny for every time the second case of an ace attorney game had the murderer turn out to be a nurse who shot their former boss over a medical malpractice incident from a while back and framed a teenager from a well-known/infamous family for it, and the case also introduced a german prosecutor who was/is a teen prodigy and wants take on the defense attorney that took down their brother, I'd have two pennies, which isn't a lot but it's weird it all happened twice
I'm not very good with my words sometimes BUT! I have a lot to say about the version of Gavin that lives in my head. My headcanons about who he is and his motivations
In my canon Billy's "Back From The Dead" ability effects everyone and most dimensions regardless of his presence in them, it was something he was inherently born with
When he worse born people began coming back to life without explaination, it was gradual at first but eventually it became just "a fact of life" that when you died, you'd be back, it's very rare a person would permanently die
Gavin has grown a fascination for this immortality, but also a hatred. He's spent many years researching how to stop this immortality plague, and eventually creating a "perma-kill serum" as a solution
This was only possibly due to him happening on a strange green stone, and he used it's powers in the making of this perma-kill serum
This was not for any noble reason though, as ever since he was a child, he'd felt like the "lesser twin" between him and Toast
He became bitter and jealous and angry, wanting to take Toast's place and be him, be the one that's admired and loved by all, he wanted to create a "solution" to immortality so that he can kill Toast and take his place, and live on pretending to be Toast, a more successful and better version of himself
I think he has a lot of trust issues, from a mix of just not being very well-liked by people he meets and subsequently coming to expect that from others, and from an idea of mind that he tends to get backstabbed a lot by people he works with, thinking they are truly a benefit to him and his own plans when really he's alone like he'd always been since he was a child
Someone asked me for Gavruss scenarios and I'll get to that, but I want to infodump about the dynamic first.
Gavin starts the show stuck in a positive feedback cycle. The experiences he's lived and the way he was raised formed his beliefs, his beliefs affect the way he treats people, the way he treats people re-enforce his beliefs. His parents died in a tragic meta accident; Michelle told him it was her fault for being an out-of-control meta and that metas are monsters because people are inherently bad; Gavin is rude and somehow racist to both sides of the meta coin at once; this makes people (reasonably!) snap back at him (like Isabella), proving his point in his own mind.
Russel is kind and forgiving to a fault. Cassie Shoal is the perfect example of this. She tries to kill him multiple times, he saves her life, and when she says she still has to hunt him down, he doesn't put up much of a fight about it. It's one of his biggest flaws— but it's also one of his greatest strengths. Russel is the only person able to get through to Gavin, listening patiently to his side of the story and pointing out the flaws in it gently. He makes Gavin mark Insecure; he kicks off that arc of doubting Michelle's teachings. (Important to note that Benji is the other person to show Gavin kindness, and this becomes REAL important later. Vika doesn't count; flirting bordering on harassing is not kindness.)
Russel's lawful good boy -> ACAB arc is one of the most well-written slowburns in any piece of media. And AGAIN, it's through his kindness to Gavin, his desire to work things out peacefully if at all possible, that make Gavin start going down the same path. Gavin starts questioning the police's motives and thinking critically about Valor Force.
Something important to me about the Dynamic (TM) is that the seeds of gay were there at the beginning, and that started Gavin's willingness to listen to Russel and thinking about what he said, but after that Gavin's arc took off on its own. It's NOT a matter of "love fixed him," but a matter of "love nudged him in the right direction." I think Gavin fixed himself, with a lot of hard work and sleepless nights wondering if everything he's ever believed was a lie.
It's also just an objectively very funny ship. We haven't had a symbol of gay stronger than the Boltcutters™ since RadioShack.