There are literal monopolies, and there are companies with monopoly power. Either condition would qualify for them to be called a monopoly. Steam(Valve) would only qualify, if at all, as having monopoly power. I make this distinction to make sure we're on the same page.
The courts have no specific percentage of market power required to hold monopoly power. The only guideline is that it usually doesn't happen below holding 50% of the market. However, the requirement can be much higher. The key here is that it's at the courts discretion. So while it may not technically be correct when I said they need to abuse dominance, it does need to go before the courts which would mean there would likely be an accusation of it.
On top of market percentage, their leading position needs to be sustainable over time. If competitors could keep them in check, or if a new entry to the market could, then they're usually not found to have lasting market power. This would, again, have to be decided by the courts. It also doesn't mean it would necessarily have to be easy for a competitor to do it.
Both of these conditions need to be met to be classified as a monopoly power. Following that, I think it should be safe to say that they're not currently a monopoly due to the second requirement. While no competing game store truly competes, they are capable of doing so. For example, Epic's "failure" as a game store is more to do with an intentional, or unintentional, failure to provide a quality product from a consumer standpoint. So while I was technically inaccurate with what I originally wrote, as I was generalizing, this is what I meant by saying it.