Apparently my giveaway was region-restricted. What do I do?

When you generate the gift link, Humble posts a gigantic windows saying that the gift is region locked and where it works. You even have to manually confirm that you acknowledged the fact.

You either provide a working key for the winner or ask permission to delete the giveaway, so you can repost it.

You can ask him if he'd mind if you delete the giveaway and re-make it for the proper region. Most people are pretty cool when it comes to that, but there's never any guarantee.

Barring that option, you can either try to get him a key for his region from somewhere else (trading for it, etc), or you can accept a Not Received for the giveaway.

on one hand, I'd suggest you for the future to look to the Deals section in order to get for each bundle or offer a complete insight on all the restrictions, value and so on.. =) For instance, browse this thread for your giveaway: https://www.steamgifts.com/discussion/eQkQ8/humble-bundle-november-2018-monthly-bundle-incl-hollow-knight-hitman-7-days-to-die

For what regards your particular giveaway this time, you're not in a bad position given the premises and what you want to do, you clearly acknowledge that it's been an error of yours!
First of all you should ask through the giveaway page if gently he can accept to delete the giveaway, and if he accepts you have just to save the key and to file a support ticket giving the proof uploaded on sites like imgur.. obviously, if you made a giveaway and you let some people enter, it's their right (given the rules of Steamgifts) to claim the gift they won, so the winner has the right to have the game, but if you are kind and gentle and explain to him what happened, well, it happened to me 2-3 times - I won a gift and the key was dupe for instance, it was my right and I could have required the key, but it was 100% great for me to accept to delete the giveaway, no problems at all.. this website is funny and entertaining, it's not a matter of life and death!

on the other hand, if the regular winner does not agree to delete the giveaway, he will mark it as not received - I got two not received on my very first days here and 0 for the subsequent months and year, since I was still a total newbie and I messed up so much that I still remember what happened!! XD But really, 1 or 2 not received aren't something that will take you away anything.. the most important thing is that you learn from the errors you made (and remember we're still talking about videogames, not health related topics or similar things..) and the next time you'll make a valid one with all the information! =)

anyways, since the winner and only the winner can see the key, you still have that key non-used and can make a new giveaway of it, your last option would be to try any other way to trade and provide him a gift or a key available for his region, but I recognize this can be hard to achieve.. so if you can't find a way to have a key acceptable for him, try to ask for deletion and otherwise next time be more careful =)

Happened with a game I won once. I just contacted the gifter and we had a polite talk and I had no problem accepting a reroll, it's just a game, and we all make mistakes.
The gifter was generous enough to "compensate" me with a Sniper Elite 4 key. Rules say you can't give the winner another game than the one that was in the GA but maybe you can get him to accept a reroll by offering something else?

Regular Dead Island is basically banned in Germany. You can buy him a ROW key that will activate there anyway.
https://www.fanatical.com/en/game/dead-island-definitive-edition