alt3rn1ty: The place to keep an eye on is the steam depots ..
https://steamdb.info/app/1091500/depots/
Where it says "Last Record Update" shows when something about the game changed.
But its not always just game files get changed, it could just be tweaks to the games description, what features it has etcetera. Currently the last record was 2 days ago, but that was not game file updates.
If you scroll down the page to where the "Branches" section is, those are the ones to watch. Most recently updated will jump to the top of that list, so when they start testing branches again, pretty soon after there will be a public release, and thats when the steam version of the game goes live, followed pretty quickly by the GOG Galaxy version of the game which uses the same files.
The only delay after the public release goes live for GOG users, is the Offline Backup Installers which take a day or so for GOG staff to make the manual installation files.
Edit: PS you can do the same for any game you are watching, just replace the number in that depots link with the number from your game page on steam, so for instance No Mans Sky ..
On steam -
https://store.steampowered.com/app/275850/No_Mans_Sky/
NMS Depots -
https://steamdb.info/app/275850/depots/
275850 is the game id.
So find your GOG game on steam, and copy the game ID number into the depots URL
https://steamdb.info/app/******/depots/
Not all games companies using steam depots stick to the same format of depot naming, but when you have seen them update a couple of times you will see how they do things. Generally you will see testing files, then maybe a Beta test will go live (if you have enabled beta testing branches), and then given a bit of time for users to give feedback on the beta then the Public version of files go live when the company is happy they have fixed the objectives for the update.
Yeah, I'm playing Watch Dogs Legion and the world is so much more alive, it actually hurts.. I will come back after thuis next patch, but Steam forums, outside of being a toxic cesspool got everyone excited earlieer today and said it had dropped, it hasn't yet, so i found that article..