I saw it via their newsletter email. Here is the link:

https://www.gog.com/promo/20161130_special_promo_the_witcher_3_goty_upgrade

They sent me the GOTY edition a few hours ago, a couple hours before the support team replied.

Guess I'll wait, because I still didn't get it. I bought both expansions before this sale though. I don't know if it matters, but maybe...

Edit:
Got it! Kewl :D

Thanks mate. Youre a legend!!! I was looking all over for that yesterday.

Thank you GOG for the upgrade! You guy's are awesome! And for the record it takes time and money to integrate a game and expansions into one file (coders don't work for free). Just be happy you can still get DRM free games at an affordable price!

YES!! Thank you for the Upgrade.. however i have a question, I see that it is listed as a separate item on my library.. does that mean That I have to download the whole 36GB again? is there a way to Just Upgrade the current version that already is on my computer?

I logged into my GOG account today an guess what I found. A FREE Witcher 3 GOTY game. Thank you guys for being persistent in asking GOG to give all owners of the Witcher 3 and all DLCs the GOTY version upgrade. And of course great thanks to GOG and CD Projekt for listening to their customers and fans.

Thanks GOG, for the free upgrade, this was a nice surprise. You guys once again, are cool guys and girls, thank you.

Just found it in my account thank you :D.

I was just wondering if this has the free 16 dlc as well?.

Also any reason to download this if already got normal installed and latest patches?.

I own TW3 and both expansions already.
I got the GOTY Edition yesterday.
Thank you very much. This is why i love you guys so much <3

Achievements don't carry over? Scandalous!

Ok Please accept my apologies GOG but you did exactly as a respectable company should have done. I am actually a 45 year old man with many games from Steam and now GOG and I am impressed with the way that you have not only taken care of me in giving me this game The Witcher 3 The Wild hunt GOTY edition after I have complained just a little bit over some wanting to pay for this game after they had already purchased it. I thank you so very much GOG and you now have my Fealty in the world in gaming. I Salute you GOG. YOU ROCK!!!

I just wanted to thank you GOG for the fantastic free gift of The Witcher 3: GOTY Edition. It certainly was the right thing to do in terms of customer service for those who have purchased all the parts of the Witcher 3 separately. Where as most companies wouldn't do this, you do did - thank for your providing excellent customer service and DRM-free games at great prices!

You will always get my business over any Steam product. Sincerely, thank you!

Even though i said it already, but let me say it again: THANK YOU GOG for the free gift! This was a real godsend, if i ever saw one. My normal game, which i got in since day one and kept slowly installing over and updating, got malfunctioning (terrible speed, movement lag, walking and animation problems)... Now that i uninstalled it and added the GOTY version instead, it works like a charm, with great speed and seems to have a better performance overall! THANKS!

Also, people, don't be bitter and sad! Your perceived "Whining" and "Gimme" from people around here, ended up in some people getting their problems fixed actually, not a childish stunt and show that you erroneously perceived! Besides, it costs them nothing at all! And feels like a great return gift to those who preordered (i know i did)!

Now, only the game guide in game's extras in library remains to go, much like 1 and 2 had one... Whee!

This goes without saying... Or even better, in CAPS and BOLD!