Some games not included in this list that have serial keys:

Steel Division 2
Battletech
Age of Wonders Planetfall
Gamedec
X4: Foundations
Imperator: Rome
Prison Architect

Btw: this post:

https://www.gog.com/forum/general/gog_games_with_serial_numbers/post1

and this post
https://www.gog.com/forum/general/gog_games_with_serial_numbers/post234

and this post
https://www.gog.com/forum/general/games_with_serial_keys/post1

and this post
https://www.gog.com/forum/general/gog_games_with_serial_numbers/post240

are a nice help for all those people who want to check, whether they have valid serial keys for all their games.

Because, apparently, there have been some issues lately:

https://www.gog.com/forum/general/doom_3_2004_serial_key_information/post2

https://www.gog.com/forum/general/doom_3_2004_serial_key_information/post16

https://www.gog.com/forum/general/doom_3_2004_serial_key_information/post17

https://www.gog.com/forum/general/doom_3_2004_serial_key_information/post20