Challenge him to a game, winner takes the loser's stake in the company.

They're probably just taking a break. time_sunk != output.

I "waste" time at work quite often but that's because I know when I am in certain moods of productivity. Forcing myself to work when I'm not in the mental state to do so is like beating a dead horse. Perhaps your co-founder also recognises this.

Have an honest discussion. Keep your concerns free from judgemental language; for example: "When I see you playing video games it makes me concerned about what your priorities are and I want to have a chat about it." If they're reasonable, they'll be straight with you. If they ascribe any judgement in your words (assuming you use non-judgemental language) then they're probably feeling attacked, in which case, I would de-escalate. The key thing here is empathy. If you handle the situation with decorum, and they do not, then you likely have more problems beyond your co-founder wasting time.

edit: coworker to co-founder

Personally, I would ignore it. If they're an over-performer and taking a brain-break then it won't affect their productivity. If they're a slacker anyway then it's a matter for management to solve. The larger problem would be if the person is an under-achieve, management knows, and they do nothing. In this case the video game playing is the symptom of an organization with management issues.

In the office? When he/she should be working? During board meetings? At home? Are you peering in the window? Is he/she winning?

If it’s a gaming startup I’d say they’re hard at work testing product! Otherwise I wouldn’t care whether it’s gaming versus _insert any other non-work activity_ unless the work isn’t getting done.

If it’s not getting done I’d focus the inevitable conversation on that metric, not the gaming.

Sounds like you are asking "What if my co-founder doesn't do everything exactly the way I want and I would do".

If you had asked, what if my co-founder is not meeting his/her obligations, then that is a different story regardless of whether they are playing video games or not.

Maybe have a vesting schedule for co-founders, and have concrete KPIs and deliverables.

As opposed to eating vegan sandwiches?

Fire him/her!

join for some gaming for bonding and change of mood, let off steam if you need and get back to work!

also sleep well :)

Context matters

Make them ceo