How much revenue does Gog make compared to valve for instance?
I don’t know. That’s not relevant to the definition of a monopoly. To be a monopoly you need to be the only supplier of a product or service. If there’s competition, but you have the majority of market share, then you’re merely dominating the market, which is not illegal (or even necessarily a bad thing, as evidenced by Steam).
Google Play and Apple store are monopolies because they are not competitors to each other, if you own a phone you’re stuck with one or the other with no choices. They also take steps to prevent competition, which is highly illegal.
See my other comment. You appear to have confused a monopoly with a dominant market position.