Key didn't work. No way to gift.

You can check region restrictions by looking at the packages in steamdb there usually isn't a "just-US" region.

There are no region locked keys for this game ;) If you go to packages you will not find PurchaseRestrictedCountries parameter in any of them ;) For comparison Capcom is known to put region restrictiopns on their game and if you go to same page for capcom game you will see separate infos for each restriction, and you will see PurchaseRestrictedCountries parameter if you go into them ;) GG probably just adds this info by default, so in case they received region restricted keys not knowing they could not be refunded anyway, because by default they will warn you they are restricted whether they are or not ;)

Gamersgate says the region restriction for every game I purchase, but I have gifted those games to other regions though. Might be to cover them if a key is indeed region locked.

If you have access to it, tremorgames has also copies of the game for nearly nothing.
Of cours there is a slight possibility that they got their keys from groupees, but I doubt it, there are more likely from otakumaker.

It's not just greenlight bundles and it's not the only case. Groupees have several problems like these with rogue developers and they never stood by their customers. They earned money so they do not feel like wasting this money to pursue legal action against dev who broke their contract, even if they are legally right. And you as customer cannot legally pursue key from the dev, because you bought it from groupees, not dev himself.

I've had 3-4 similar problems with groupees in the past, and not always greenlight games. Nekro was offered as normal game in BeMine bundle, but developer decided to deactivate keys "because game stopped selling on Steam". Groupees reaction was the same - please contact developer and sort it out with them.

ofc they can. Groupees or any other bundle site signs a contract with developer on him providing keys, by deactivating keys he is breaking the contract, and groupees can pursue legal activity against dev. The fact of the matter is thou they don't feel liike they need to. They already got their money, so did dev, legal action costs money and even if they win legal action (they probably would) it's not porofitable - only person paying for legal action is groupees and dev, only people profiting from it are customers, so why would they bother? Also you as a customer could pursue legal action against them, but again, who would bother for a few cents worth key?

So anyway, tl;dr, it's not what you say that they can't really do anything, they can but it's not worth it for them.

Another option is using Steam Market (creator gifts winner some very cheap item, winner sells it back to them for the price of the game). Of course the first risk you mentioned stays the same, so I'd probably only try that if either the winner is trustworthy or the amount of money is small enough.