GOG Galaxy suggests leaving a review after 5h of play if I remember correctly. It's always a problem to have uninteresting reviews. But we can upvote / downvote a review to promote interesting reviews over uninteresting ones.

A filter for "at least this many characters" probably sounds most reasonable. If GOG wants to chase the number of reviews, no matter the cost, they're welcome to do so. But providing tools to skip over these "reviews" would be a nice gesture. It is obviously having a negative impact on the overall review experience and that sounds like the easiest fix. Because this is now a certain fate for any game that ever becomes a giveaway for example.

GOG- "Hi renowed Publisher, I would like to present you GOG.com, a place where you can sell your games. Would you be interested to sell Half-Life 3 in our renowed platform?"

Pub- "Hum, GOG? Never heard about it. Checked the website and judging by similar games you sell, like Portal 3, seems it's a small platform since there's very little number of reviews." ... Crap, always though GOG was a hacker group ...

GOG- "Hold my bee... AHEMMM. Don't worry, we're in the process of creating a easier way for our costumers to post reviews."

Blue- "Don't worry good citizens of our Galaxy, it's just a bug, we're on it, we called Terminator."

Honestly, why should anyone bother to write a proper review here? Even if you write something long, detailed and well argued, your review is likely to be downvoted to hell and disappear to the bottom of the pile because people around here can't tell the difference between "unhelpful" and "I don't agree with this". And conversely, I've seen one sentence reviews just stating "this is a classic" or something like that or describing a single technical issue among "most helpful". It's why I stopped writing reviews altogether.

It dose not seem to effect all games. Metal Slug Tactics has some pretty decent reviews going on.

As has been stated, it is likely a Galaxy issue, and also probably related to Amazon Prime, where gamers who haven't used GOG before or rarely, just use Galaxy when they do, and have more interest in getting rid of the nag screen, than actually giving a proper review.

Play games you like and ignoring reviews totally. That is how it should be doing. Enjoying games and love and live and laugh.

Something like that.
The link Dark_art_ provided does contain a foretelling post by Time4Tea:

Laziness.

Well playing Parkitect with gog galaxy and the first 5 hours it showed if i want to review the game, i clicked later.

Then after 10 hours it asked me again if i want to review the game and there was a button don't ask again for this game or something like that.
Don't know if the don't ask again button was there at 5 hours of Parkitect with gog galaxy.
Also never did i write a review the only thing i do is give stars sometimes to games on gog.