Martek: Actually, I would not agree.
This thread and plenty of other threads over the years, here on GOG, debate on that very issue. Thus, no, I don't believe that assumption (they "own it on GOG") can be made.
That's a good practice. Just keep in mind - not everyone does things the way you do - it's good to be cautious about recommended practices based on your way of doing things as if everyone does things the same way.
I'd probably still argue against "automatic" things - I tend to not like them due to "unforeseen circumstances" arising.
But, if it was noted with a confirmation; then at least one could go forward with foreknowledge about that implication of their submission. I'd grudgingly accept that.
I'm still not sure I understand your issue here. As wvpr noted, if someone posts a review of a game here on gog, that means that yes, they own it somehow (whether on gog or not), or have played it in the past. Your argument of "it will give SJWs fuel to attack the reviewer!" makes no sense, because the very act of posting the review is already all the fuel needed if someone felt like attacking.
Scenario 1:
-Person A posts a review of a game on gog. As is currently the case, there is no indication of whether Person A owns the game or not
-Person B attacks person A because the game is a controversial game on gog.
Scenario 2:
-Person A posts a review of a game on gog. Either a rule has been implemented that only people who own the game can post reviews, or there is an indication that A owns the game in their review.
-Person B attacks person A because the game is a controversial game on gog.
Neither scenario is any different in terms of attack fodder.
Now, as a completely side topic from this discussion, I too would consider it better if only those who own a game on gog (perhaps even only those who have downloaded the game from gog) can post a review of it. It is all very well to post reviews of games you played ages ago, but that experience may not necessarily be representative of the version existing on gog. The gog version may have technical issues not present in the original version, or it may have fixed some technical issues that had always existed in the original version, or it may be played through a wrapper/emulator/VM that changes the experience somehow.
If it isn't possible to restrict reviews to only people who have got the game on gog (perhaps because of the issue pointed out by wvpr that a game wouldn't get any reviews due to being old and obscure otherwise), then yes, an indicator that the person who wrote the review owns the game or not seems prudent.