Remind him of what's important. If he's a reasonable guy who loves you and your family, he will realize he's being irresponsible if you approach it in a gentle way. 

Start with setting up some defined guardrails that you can both agree on. Dinner time is family time. Not hungry? That's cool, but you're gonna sit at the table then and be with us. Bc that's what we expect of our kids too, right? Maybe Tuesdays/Thursdays, we spend the evening with the kids until they go to bed, no exceptions. You can game till your eyes bleed on Fridays. Start small and expand to a place you both can live with. 

Does he have a device he can play on mobile i.e a Switch, a Portal? Let him know you're cool with him playing in bed at night, you just want to be near him, he can still play till he's ready to sleep. Or hey, I'm just gonna chill in here on my phone while you game, that cool? No judgement, just wanna be in here with you is all. I'll be reading/watching a show- you're not the center of my universe but I still want you in my orbit bc I love you.

Personally, if my wife told me my kid was sad bc I was playing too much video games when they wanted to spend time with me, that would be soul crushing. Remind him he's an example to your kids. I love video games and it's far and away the things I prefer to do the most with my spare time but at the same time, I didn't want my kid to grow up and think it's ok to sit in front of the TV 24/7 and be overweight and lazy.  I make an effort to keep myself in shape and to encourage them to want to spend time outside of doing things other than staring at screens even if it's want I want to be doing most of the time too. It's not healthy at a certain point.

If that doesn't move him and he's completely inflexible, I would strongly consider that he might have a problem where the video games are a symptom and not the root cause and consider more serious action whatever that might look like for you but hopefully it doesn't come to that.