You have a great memory anon, something does indeed happen between them as teens :3c I'll keep the details to myself but Gary ends up in locked-in syndrome. Mikael is the first person to notice that Gary is aware and not merely comatose, which thankfully does lead to him getting the appropriate therapy and recovering some motor functions as he reaches adulthood.
At first it's complicated for Mikael to visit him but Gary's mother eventually helps and he gets to come and go as if family. In a way she kind of accepts him into the family too even if he's so outside of their circle. Mikael takes care of Gary when he's there and continues to do so even after he's discharged.

Their relationship becomes complicated, there's lots of resentment from Gary and at the same time Mikael remains his only means of escapism. Slightly sci-fi but these characters were first created within the context of being able to venture into other people's dreams thanks to an experimental drug and I still like this idea,,, experimental dream therapy designed to help coma victims, the last place where Gary can still walk and talk and taunt Mikael's guilt, always taking the form of himself as a child even as they grow older, the return to reality always hitting like a brick. Lots of fun but purely hypothetical, for now I'm having fun daydreaming about their teen years before the accident