I was equally gaming obsessed at around the same age. For me it was world of Warcraft. My parents were also concerned, but they showed interest in what I was doing and I explained it to them. When they realized I was leading of raiding Guild of over 50 people, and a good chunk of the time was actually me managing people, including resolving conflicts and all sorts of administrative tasks, they weren't nearly as concerned.
Depending on what game your son is playing (I've heard of Roblox but I'm not familiar with it), and how they're playing it, it very well may be that your son is developing some very valuable skills. It might not be managing people, but it might be creative problem solving, setting goals and working towards them, etc.
It sounds also like your son is a bit obsessive, and clearly very intelligent if he's able to achieve the grades he has while devoting so much time to a game.
I was this way too.
By the time I was 23 years old I tapped into some self motivation and desire for more financial freedom and became an entrepreneur. Through the rest of my 20 isn't into my early 30s that obsessive attention shifted from gaming and into my business. I didn't make millions of dollars but that type of focused effort led me to develop many valuable skills. I ended up selling my business for a small sum and now I make good money consulting on my own terms.
Frankly I think the most constructive thing you could do is just show an interest in what your son is doing in his game. In order for this to be constructive you have to reserve judgment. Don't hope for any specific answer just be curious and empathetic. Rather than reading about "gaming disorders" read about Roblox, maybe even try playing it a bit yourself, and try to really understand what it is your son is doing and why.
You might find in the end that nothing is wrong.