is there any way to un-redeem a gift?

Nope.

steam support.

but I think they'll only do refunds/takebacks once.

Steam does refunds extremely rarely. You must have the best reason in the world for them to do that.

Well, the re-packaged this one collection of games I had opened. Maybe the support will help you with this too.

Use a time machine and go back in time before you redeemed it.

As gift you won't get it back. If you ask steam support to remove it, than it's gone. You WON'T get it back as gift!

You didn't really like it after .1 hours? Well, I suppose there just isn't any content after early game in survival games.

Support probably won't give you a refund just because you didn't like it.

I'm pretty sure they don't refund you if you alraedy played the game. But if you did not play the game yet and you are in the EU you can ask for a refund, because EU law override Steams ToS. However, this depends on the quality of the support bot.
Also you can always refund preorders. Also you can always ask them to delete a game from your library (without a chargeback ofc).

Ask Elizabeth to open a tear

As a general rule, they won't do it.

Maybe if you can come up with a very good reason, or if you are lucky and find a support guy in good mood you can have it back, but I wouldn't count on that.

It depends, which country are you from ? If you're from EU you can just quote the law and they'll do it. They won't restore the gift, maybe just refund your steam wallet.

Yeah I should sell my library so I could get new games then sell them again after playing. Good idea.

I imagine for exploitative reasons, they may not have a way to do that...