Indeed, if I had a game removed, unless I got a notification I wouldn't know if it was missing or why, I think it would be a dreadful policy not to inform somebody if/why a game was removed.

Legal question. Who is it that they should notify, the one buying and paying for the item, or the one using it? Case here is that I buy a game with fraudulent means, then give the code to you for use. You won't get any notice of the code being revoked, since you don't have anything to do with the obtaining of the code. Whether I get a notification or not has not been cleared.

And if you want a physical world example, I buy a car for my son to drive. I can no longer pay the monthly payments needed, so the car gets repo'd. What notice will my son get, other than not finding his car?

I found the post I was thinking of, here.
Just in case you were curious.

The difference is that GOG is perfectly aware of who is using the game, as well as who bought it. Indeed, they have to be aware in order to remove the game from the account.

As to whether or not they are legally obliged to notify the user, I have no idea. I think it would be the courteous thing to do though.

When such screenshots hide the gifter, there's already something fishy going on XD

No, not at all. I would also hide any personal identification as a matter of principle.

What I mean is that on no site that I know of would something like this be done without a notification. And the reason for that is because people do have legal rights. They can't just take games back without explanation or notice, that's stealing and in cases where it turns out to be incorrect there's liability issues.

I have over 150 games in my library if they were to take one out of my library, I might not ever notice as that's a large number of games to have. Or I might not notice in time to do something about it. Assuming that I'll remember the games I buy during sales and then expecting me to notice if ones missing is predatory. I keep records, but checking dozens of emails to see that the games I bought are still present is a pretty substantial burden on the user.

It's not expecting them to cover end user's judgment, it's expecting them to behave ethically. Part of consumer protection is making sure that the customers are aware when things are done that adversely affect them.
Considering that they know who the account owner is, that's where they would send the notice. AFAIK that's how other services do it. People complain about Steam, but don't they provide notices when they take games back?
That's the thing, whether they've discovered a technicality that allows them to do it or not, it's not the kind of behavior that we've come to expect from them. I don't think anybody expects them to let people keep games that weren't paid for, but the idea of taking one back without notification, especially on a site where games go missing fairly regularly, is ridiculous.

Without notification the customer has a greatly reduced likelihood of protecting themselves.

As far as I know, GOG keys reselling is not allowed.
So *any* GOG key bought outside of the GOG official website is an illegal sell.

Not "barely legal", not a "shady deal", not a "grey zone", but a clearly illegal sell that will get removed from your account at some point.
Don’t buy GOG keys outside of GOG and you’ll avoid being screwed. If you still have doubts about a sell, contact GOG officials *before* committing any money. Better be safe than sorry ;)