Most laws don't have a problem with becoming a monopoly, the issue arises when you use your monopoly to stifle competition. Do we have any evidence of that?
Also, given how copyright works, no one sells games, only licenses. The only games that could be sold are public domain - even FOSS software is license-based. Now, if you mean that the way Steam works that you can most often have access removed from a game after you've purchased it, that would be a more correct statement with competitors who behave differently.
As for Gabe's influence, he set the direction and attitude of Valve. That isn't a day-to-day operation, but is vital to the operating principles of the company (which includes loot crates). This is no different than the CEO (or whoever) of Costco insisting those hot dogs can be bought for cheap. That is also not a major part of what he has to deal with while actively running the corporation, but everyone knows about it.