How bad is this game, that it's been free so many times? :D

Apparently, it's pretty decent.

I thought 3.5 stars was the lower end of the gog spectrum?

Well out of five that's pretty good, out of ten it would be worse.
It's still not MOO3 level bad.

It's nice, I played it a bit, then I got bored.

Yeah, gog community is a pretty polite as far as ratings go (or gog itself is good at weeding out utter trash), but you have to adjust for that: There are no 0 or 1 star games, there are only four 2 star games in the entire catalogue (MOO3, Gothic 3, Slender: The Arrival, and Omerta), twelve 2.5 star games, and so on.

So if 2 is 0, and 5 is 100%, a 3.5 star game like Pid would be at 50% :D.

I went ahead and claimed it. It's MetaScore is 63/100, so your GoG conversion estimate is pretty good!