GalagaGalaxian: It should also be limited to people who have already bought the game.
Part of actually owning our copy of the game (disregarding that its EULA dream logic may claim otherwise) is that we are free to give it to somebody else. We may not have permission, but an essential component of freedom is not needing permission. So there is no way of knowing whether or not the reviewer owns the game. And there shouldn't be any way of knowing. It's not our business.
The obvious solution is moderation of the reviews. Numerous ways of handling this have been absolutely perfected over the past decade and a half. Many of them are opt-in, decentralized, personalized, or all of the above. The only reason we don't have these options is GOG doesn't bother to make them available, because we don't need them, because they are just game reviews.