I can confirm that, on my systems, a) games that where installed before Galaxy, those aren't registered there (unless added manualy or by a folder scan), b) With a few test (around a dozen games from various times) installs, all current offline installers I tried register the game within galaxy and c) I tried some really old backuped installers from around 2015 and those wheren't registered. So I guess all this nice little notes in the changelogs (if there are any) stating something like "[WIN] updated internal installer structure, no changes to the game files" could well mean "Hey, we're checking for the presence of galaxy during the install and register the game there for you" :(

The least thing GoG should do IMHO is to make that an option during install, as I see some major culprits here. One being as an example: What if our household has 2 or more GoG Accounts (one for me, one (or more) for the child(ren))? But as the systems belong to me, I also wanted to play MY games on those machines where Galaxy is installed (for Multiplayer) for other family members (on their account) and now what? MY game is then also registered in their galaxy install? Oh well, add another user... sure, one solution, but at first that shouldn't be needed and my decision, wether I am going to admin another account on my machine for one particular cause, or if i prefer the convenience to use just one account (and keep the OS partition free from (even if it may just a few MB) any file system burden/deadweight and administring obligations... Sigh... )

P.S.: If anyone knows/find out HOW the installer regocnizes the install (path can't be it, as that is (at least one Pro) changeable during install), so hopefully just one or a few registry entries - if thats the case we may well write some script that simply renames that key to something irrelevant and back if we so desire (before/after running an offline installer).

It seems that you, Sir, just delivered us from the evil of forced Galaxy integration. :D

Renaming said key in HKLM to the purely arbitrary name "GalaxyClient.disabled" did the trick. I tested it by uninstalling two "offline" installed games via the Galaxy right-click menu, one with kept local savegames and configuration, and one with deinstalling everything. Then, I renamed the Galaxy regkey and reinstalled both games as well as another which never was installed before. None of them were being registered in Galaxy! \o/

So, it seems that this problem is solved – for now, as it remains to see if future updates of Galaxy or the "offline" installers will change it back to the old behaviour.

I concur with your proposal of a clearly visible option for this in the "offline" installers.

Thanks to all for participating!

Oh, and if anybody discovers how the installers tell Galaxy about the games, and maybe even how they can be removed from Galaxy without reinstalling them, feel free to share it here. :)

I suspect that once a game has been detected by Galaxy the installation is normalized between what an offline installation offers and what a Galaxy instalation does. This is, it should be irrelevant if it has been installed from Galaxy or from an offline installer. The sense of this should be the functionality with or without Galaxy.
So the real question should be how you can remove a game you own once you have Galaxy previously installed.

Obviously I am not completely sure but to me it makes sense.

Greetings.

Oh, forgot, your (the OPs) original question... So far my observations in this regard are:

The Offline Installer is creating 10digit long keys (I assume this to be gogs unique game id) in this registry path: HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\GOG.com\Games. This is indepentend to galaxy and also happens if isn't installed.
(Below <path_to> refers to : OS_Partition:\ProgramData\.)
Checking the timestamps in <path_to> after running an offline installer: all files within <path_to>\gog.com\galaxy\lock-files and some (galaxy-2.0.db*) within <path_to >\gog.com\galaxy\storage are updated and also within <path_to>\gog.com\supportInstaller a directory with the name of the corresponding registry key (what i called gogs unique game id above) is created, containing two files (one *.info and one * .script).
The files within lock-files are only containing a 4digit number, and I am inclined to believe that those file hold no relevance in regards to the game installed (also with <path_to>\galaxy\communication_config.json (also updated timestamp) I am not seeing any game related information therein). But I guess\assume the *database* files within \storage are updated with the relevant info and therefore I am inclined to say that deinstalling the game via galaxy is (for now) the only reliable\(user-)feasable way to remove it from galaxy (and reinstalling it afterwards whilst the registry key with the path to galaxy information is renamed, so that galaxy ain't updated with the information).

Sorry,
Sadly, i wouldn't dismiss the topic for now and won't consider it (or the underlying problem) solved :(

As I noticed, after playing a game that I had installed before Galaxy, that I got some achievements for it. So renaming the registry key to the galaxy path just stops galaxy from recognizing the game as installed [edit: better to say, it just voids the installer being able to enter the game into galaxys local database]. Well, some quick/unreliable tests later - renaming each *galaxy*.dll in its folder didn't changed that behaviour (no reboot, so the dlls could still have been loaded in memory?), disabling the gog services and blocking the games executable within the firewall (all trafic) weren't helpfull either... (Backing up my system partition and) Creating another user in windows, sigh... galaxy ain't installed per user (neither are the offline installer games I had installed), but i wasn't signed in to my gog account in it (at least galaxy stores that information per user) and playing the game in that state didn't got, as i checked later on my regular user, me the earned achievements in galaxy. So, (writing back the previous partition image) I just signed out in galaxy under my regular user, and ... voila, no achievements gained in galaxy (The game I used (War for the Overworld) manages the achievements ingame, so i was aware when and what achievements I had reached/gained).

I guess, that signing out doesn't stop the "communication" (by whatever means) between the offline installed games and galaxy (and I still would like to know how exactly galaxy get that data if it ain't running), but well as it does not have any account associated with that session.... . Still, i won't consider this as a good solution :/

P.S.: And now, more then before, I think that there are problems that propably can/will arise on shared (multiuser) systems and the use of multiple gog accounts in conjunction with the use of offline installers.

One thing to heed regarding the registry edits: Windows 10's "feature updates" are actually re-installations of the whole OS that are known to reset manual edits to the registry.

So, the renamed keys to prevent Galaxy integration of "offline" installers may have to be repeated after every W10 feature update. Can anyone confirm this?