I just tried redeeming a code on GoG Galaxy 2.0.26, but was told my code was invalid.
I know the code isn't invalid because I then tried to redeem it directly on gog.com and it worked.

The code I tried to redeem was 4x4 characters (with hyphens as separators), and it was a code for S.T.A.L.K.E.R.: Clear Sky

I'm also having the same problem. Coincidentally it's also about a S.T.A.L.K.E.R. key.

I made the 0114290 incident on the GoG bug-tracker regarding this issue. Unfortunately it was dismissed as being a support issue. It is not a key / code issue as I've already mention. It's a client related issue. GoG Galaxy is clearly not working as it should. It either has problems with the reCAPTCHA system or is pointing to a wrong end-point. Sadly, I don't have rights to reopen issues or to leave comments that's why I'm leaving a comment here.

Here are two screenshots using the same key. Key format is XXXX-XXXX-XXXX-XXXX. The key was redeemed online a couple of days ago. You can see that I own the S.T.A.L.K.E.R. games from the client screenshot.

After entering the code in the browser section, I'm given a reCAPTCHA puzzle. After finishing it, I get a confirmation that the key was used (which is correct). Doing the same thing from the GoG Galaxy 2.0.36 I get an error stating "The code doesn't seem like one of ours", which is incorrenct. I'm not getting any reCAPTCHA puzzle in the client. The client should also give me a message that the key was used. (in my particular case)

The code was pasted and hand-written in both cases to check spelling or other symbol differences. Same results in both cases.

Exactly what you said! I tried it on Chrome and it worked. Thank you!

I tried to Redeem 2 codes to Activate the 2 Witcher 3 DLCS, and it says that I have to own the game, witch I do !!! anyone can help??

According to your profile, you played The Witcher 3: Wild Hunt - Game of the Year Edition yesterday. That version already includes all DLC. You won't be able to redeem any DLC because that's for The Witcher 3: Wild Hunt (the standard edition).

Check your orders history and hopefully you'll find an "Ask for refund" link below each item. It probably won't be available, since the items were delivered as gift codes, and so you'll need to open a support ticket instead. If more than 30 days have passed since your purchase, point out that the store incorrectly doesn't mark the unusable items as owned ("In Library"), so the accidental purchase isn't your fault.