My SO and I have been together for over a decade. He's always been an avid gamer and I like games on occasion too, though I don't have much time for them with the kids and such.
I stay at home with our four kids because, with our son's extensive medical needs, it's not feasible for us to both work while keeping up with his (sometimes 3-5 a week) appointments and any caregiver he has needs to be strong enough to lift him in case of emergencies or falls (he has cerebral palsy). 
SO works a 9-5 job in IT, but brags frequently about how he has time to "dick around" (his words, not mine) and work is boring.
When he comes home, he is usually around the family for an hour or so, then he goes to his computer (upstairs) to play Path of Exile, sometimes for 4-5 hours at a time.
I don't mind caring for the kids and I take pride in doing all the cooking and cleaning, but I get really lonely and the kids barely get to see their dad. It would be nice if he'd read a story or take them to go get ice cream or whatever, but he'd rather be playing games than anything.
He's stopped fishing, going out with friends, and any hobby besides gaming.
I've tried to ask if he's feeling stressed and I've watched for symptoms of depression, but I'm getting nowhere.
WIBTA if I just told him I'd rather he limit his gaming to later evening or a certain number of hours a day? I feel like he's a grown man and should have autonomy, but I'm concerned he's missing sleep and avoiding other activities that might be more emotionally healthy.
I've tried having him bring his stuff to the family common areas, but it just ends up back upstairs. 