The game finally showed up for me. Looks like it was taking long time to be added

Still nothing.

Simply looks like they adapted their redemption system to handle the massive influx on Thursdays, as Amazon users rush to register new games. Instead of the server freaking out and throwing up aggravating Captcha puzzles and error messages like it's being DoS'd, they've pushed the redemptions into a queue to allow account updates to be managed behind the scenes, while the onslaught continues!

If your key shows up redeemed, folks just need to chill out and give the backend a chance to catch up.

So, I am getting this message when I redeem the code for Hero's Hour.

Code redeemed successfully!
Great news! You’ve successfully added 1 item to your GOG account.

Due to high demand, your game might take a little longer to appear in your Owned Games section, but don’t worry — it’ll be there shortly!

I'll trust that it'll show up sometime today. I don't know if this is the issue people are having, but at least it's not at error message.
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Edit:
Also, try not to redeem it more than once, I did that and it went through. I was mainly thinking I needed to do it again since I didn't see it in my Library or on the Orders & settings page.

So, avoid doing that or we may end up inadvertently giving the system additional issues lol.
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Yep, it's here now for me as well. Took around four hours counted from when I first tried to redeem them.

At least downloading them is fast in the middle of the night when the servers aren't crowded any more. ;-)

Nice, thanks for letting us know. :)

It's likely cause the Winter Sale started and they (GOG) already posted a thread explaining the redeem system was having issues a few days ago. So that may factor into it as well maybe? Unless they already resolved that completely, in which case it has nothing to do with this.
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Update: All 3 just showed up after re-putting the codes again 5 minutes ago. very odd but yeah it even put up a warning now saying it may take a while for them to go into the library due to traffic or something.
Might be this but I did get new warning about slow time for redeeming stuff now.

Games showed up for me as well.

I just got a free trial of prime. Two questions:

What happens if I go to redeem a game I already have on gog? Will gog stop me from redeeming? Will Prime stop me? Will the key be wasted? I'm basically wondering if I have to check every game first before redeeming to make sure the keys aren't wasted, or is there some safeguard that will stop me from claiming gog games I already have.

Some of the games are on prime itself. When my trial ends will I still have access to those games or will the prime "launcher" only work with an active membership?

I had one "rogue" freebie that showed up during the night on my account, as the warning message had explained.

No warning. The key will be wasted.

Yes, you have to.

The correct answer is depends from game to game.

So to be safe you should check every game before redeeming.

You will keep them....