It's monopolistic practice is soley due to its market share, that alone is enough to. It's a monopoly that isn't anti-competitive, it's inherently not bad, as long as it isn't being Abused, many misconstrue anti-competitive as monopolistic, the term doesn't go hand and hand. Monopolistic competition exists when many companies offer competing products or services that are similar, but not perfect substitutes. This is valve at the moment with steam. Alternatives exist but none come even close to being a full substitute. but that's OK it isn't a bad thing, but it doesn't change the fact it's monopolistic.

As for the gog thing, maybe it is easier than I thought, if so I'm surprised that no other game store has done so, steam dedicated an entire division to it and it still has a lot of issues with functionality and usage.