Does the review system start at 0 or 1 stars? Or to phrase it better, when a game has 3 out of 5 stars in user reviews, does that mean 50% or 60%?

GOG reviews start at 1 (I don't think 0 stars is possible). I guess one way of looking at it when "translating" it to percentages is : 5* = 90-100%, 4* = 70-89%, 3* = 50-69%, 2* = 30-49 and 1% = under 30%. Underneath the title of the game on its page is the score, eg Witcher 3 is 4.9/5 (which is pretty much 98% equivalent).

It'd be nice if we could have a nice fractional 10 stars or even a colorbar gradient. Just place an arrow somewhere and then describe it out with 5000+ words to work with.

Since any such system is completely arbitrary, you can give as many as you want or feel is correct.

It's infinitely easier to define from a list of users the lack of stars as no rating rather than that being a rating in of itself.

What are you talking about? No you can't. If you feel 0 stars is correct, then you still cannot give a game 0 stars in GOG's review system.

Instead, you forced to give it at least 1 star even if you don't believe it deserves that star...which is one of the reasons why GOG's review system is broken.

I kinda dislike the 5 stars system, since it's way too imprecise, but, when I have to use it, I consider:

0 .. 00-19% abysmal-dreadful
1 .. 20-39% poor-subpar
2 .. 40-59% insufficient-mediocre
3 .. 60-74% sufficient-fine
4 .. 75-89% good-very good
5 .. 90-99% great-excellent

(btw I doubt real 0-stars games exist on Gog)

A.D 2044, the other Postal, Hello Tech Demo, just to perhaps posit a few titles as duffers.

This is how I interpret ratings here on GOG:

1 to 2 stars: Pure junk. Avoid.
2 to 3 stars: In most cases, poor/mediocre game. Sometimes it can have interesting gameplay elements or can be good for a very specific audience
3 to 3.5 stars: Average. Can be considered good if it fits your tastes.
3.5 to 4 stars: Average/fine
4 to 4.5 stars: Good game
4.6+ stars: Excellent game

Another advice: Don't trust early ratings. You should wait a few weeks after the game has been released to have a trustworthy rating score. Haters and fanboys are always the first to vote.

One of the joys of MaGoG was that you could easily search by rating. Fortunately it is still possible with the csv database dump.

There are a bunch of 0 and 1 averages, but viewing a random selection indicated that there weren't enough reviews for GOG to provide an average.

The first review I found with an actual star rating bottomed out at 1.2.
Of the 21 lowest ranking items
9 were Beamdog DLC. Soundtracks and portraits
2 were Simon the Sorcerer 25th Anniversary edition
another 2 portrait packs
The rest were DLC. Soundtracks, cosmetics, a free strategy guide. Nothing that could be generalised.

These are some of the most reviled releases on GOG, yet not even Beamdog could manage a star rating below 1.

If someone had ever learnt how to give a sub 1 star rating, it would have been publicised so that it could be bestowed on Beamdog.

Disclaimer: I don't have an opinion on whether everything Beamdog touches actually deserves a 0 star rating. I just know they are the community's favourite punching bag.

For me, I see 4 stars as a viable option, then I look at reviews elsewhere. 5 stars are good games. If game had 3 stars or less in here, it's going to be very rough, very political/anti-consumer, or unplayable.

50%. As you cannot give 0 stars game ratings on GOG look better than they actually are (both the rating stars and the finer granulated rating numbers). They do not exist as nobody can give a 0 star rating. So the lowest possible game rating on GOG is 1 star and the percentages on your list actually would be:
1 star: 0-12% abysmal-dreadful
2 stars: 13-37% ...
3 stars: 38-62% ...
4 stars: 63-87% ...
5 stars: 88-100% great-excellent

GOG's system still gives the user more choices to rate a game than the thumbs up/down only system on Steam. But not being able to give 0 star ratings somewhat mitigates that advantage and dresses up the results.

Interesting, but no, I don't adapt my ratings to weird scales.
If zero stars aren't allowed, it just means that the 00-19% range doesn't exist on Gog.
Not a major issue for me tho, since I've always managed to avoid such crap titles anyway.