Puntolory: I'm just guessing since I don't have the game on GOG (yet), but it's possible these updates are covering the bugfix/maingame/offline DLC portion of the updates that have been released on Steam in the last couple of years.
The Steam version patch notes have a lot of content related to things that we don't have in the GOG version (steam workshop, online party, vanessa's curse), however if you exclude all of that you can see from the notes that there are still a few changes and bugfixes that cover the portion of the game that is available on GOG too. I believe those are the most likely things that are being changed or touched on with the GOG patches.
For reference, you can view the latest patch notes on Steam here:
https://steamcommunity.com/app/253230/allnews/
Thank you for this explanation.