I have just collected all the 19+base cards of the Skellige deck, but the achievement is not popping up on GoG; the mission is completed so I'm sure I've done everything right.

I even bought the Gwent guide book to check and it says my collection is complete.

What's going on??

Achievements have been bugged on and off for a very long time and it may kick in tomorrow or next week or next update:-(

Yea, too bad! The first two I got popped up just fine so I assumed they were working..

I'll try to keep all the saves and load the one right before the last Gwent card to see if it works after a few tries...

Are you using the Game Overlay? I couldn't get a different achievement to unlock (the knight tourney one) until I turned the game overlay on. Could you try loading the save you have before winning the last card and see if it works?

I just got this achievement, so I suppose it fixed now.