1st, sorry for my english, I'm just a french speaking guy.
On my side, I bought the game back in 2016; yesterday, after reading the forums on Steams, as the developers don't bother to read the GOG forums; I asked GOG what will happen to GOG customers and if we could expect at least to have access to the final release; here's what they replied to me (in french):
Malgré tous leurs efforts, les développeurs derrière Wolcen : Lords of Mayhem ne seront pas en mesure de maintenir la parité de mise à jour entre GOG.COM et d'autres plates-formes. Nous nous sommes mis d'accord sur le fait qu'il est préférable de retirer le jeu de notre catalogue.
Si vous ne souhaitez pas conserver le jeu sur votre compte dans sa version actuelle, nous pouvons vous proposer d'échanger le jeu dans votre porte-monnaie GOG, quelle que soit la date d'achat initiale.
So basically, Wolcen is removed from GOG and you have 2 options:
1. Keep the game in it's current state in your GOG library; knowing that it will no longer be updated;
2. Be refunded on your GOG wallet whatever the date at which you bought the game.
On my side, I choosed to be refunded on my GOG wallet; and sure enough a couple of minutes later my wallet was credited with the amount that I paid back in 2016!
I'm really impressed by the fact that GOG accepted to refund an early access game that I bought back in april 2016; big thanks to them !
Regarding options for this please note that you can also request a steam key from the Wolcen Devs as below, Just a reminder to be polite the goal here is to get your steam game key leave the browbeating to the fourms and discord XD, i got my replacement key in 3-4 days witch is quite a good responce time.
Also to get your GOG receipt go to your Account Tab > Orders > search for wolcen > go in and take a snip it / picture and attach to the email that all you have to do :).
Title - Wolcen GOG Key - Swap to Steam Acc
I have been informed that wolcen on GOG will no longer be supported and to contact you, i would like to request to swap my GOG key to my steam account, Please see my attached purchase receipt.
(your gog acc name)
(your steam acc name)
(attach Pic .jpg / pdf etc.)