That is also I'd advise for people like the OP, if the lack of separate offline installer patches is really an issue for them. At that point it is probably far easier and less headache just to use a client which updates the game for you, even automatically if you want. Especially important with in-dev and online multiplayer games that can receive updates frequently, and with multiplayer games it can be important that all players are using the same version anyway.

However:

Don't GOG games generally launch Galaxy, if Galaxy is installed and you try to start the game with e.g. the desktop or Start menu shortcut to the game? Even those games that you have installed using the offline installers, not Galaxy?

So to avoid that, you need to create separate shortcuts manually to all your installed GOG games, and it is not always only about "just find the exe and create a shorcut to it", as many games also require various extra options and switches in the shortcut to run properly (like, I think, all DOSBox games need to be told which dosbox config files to use, and at least in the past Thief Gold needed a switch to tell the game that it is running in "Windows XP" (was the option something like "-ntlg" or somesuch) as otherwise it would fail to launch the game).

I would probably have it installed (and I already tried once in the past, but back then Galaxy installation would not work for me), but nowadays I don't because:

- what I just described above, ie. to my understanding my GOG games would try to launch Galaxy every time I just want to play the game. To avoid that, there'd be the inconvenience of having to create extra shortcuts manually.

- I also have a couple of GOG games installed on my work laptop, and I prefer not installing any online (gaming) clients on it, be it for security reasons or whatever. I presume an offline installed single-player game doesn't offer similar attack vectors (as installed online gaming clients do) to cyber criminals.

https://blog.checkpoint.com/2020/12/10/game-over-vulnerabilities-on-valves-steam-put-hundreds-of-thousands-gamers-at-risk/

https://www.forbes.com/sites/daveywinder/2019/08/09/critical-steam-security-warning-issued-for-72-million-windows-10-gamers/?sh=1fabb91835e1

https://www.positronsecurity.com/blog/2020-08-13-gog-galaxy_client-local-privilege-escalation_deuce/

- I prefer installing and running my GOG games with the offline installers, because then I also get to test that the offline installers actually work. After all, we've had cases where the offline installer has e.g. missed some feature or had not enabled some DLC it was supposed to enable, and those would not have been caught unless some people actually use those offline installers, instead of merely downloading and archiving them.

I recall one of the newer Deus Ex games was like that (and GOG later fixed it when they were made aware of it), and I think I also noticed something possibly with Battlefleet Gothic: Armada 1 or 2. At least it was unclear to me how to access some DLC or something, when I ran the game which was installed using the offline installer; I think I asked about that here in the forums, I need to look into that more...

- In general I just see the extra client running as an extra nuisance, when just trying to install and play a single-player game that hasn't received updates for years.

In Steam this is causing me "problems" (or at least inconvenience) already because I generally let my son use my (main) Steam account to play Team Fortress 2 (a free-to-play online FPS game) as it has more inventory space and more items (weapons etc.) and he likes to try them all out.

So I use the secondary Steam account I originally created for him, and play TF2 on it with less items and less inventory space. That's fine I guess.

But now, if I wanted to install and play some single-player game from my own Steam library (I have several hundreds of games on Steam as well), I can't download and install them with this account while my son is using my Steam account.

I guess one option would be that I logged out of this secondary account and install the game with that main account, but I haven't tested if that is possible while he is playing another game with that same account. Maybe it is. Or then at least some Steam games apparently offer some kind of "family share" option so that family members can download and install games from your Steam library, but then you have to take extra steps to tell Steam which users are those family members eligible for using your games.

Either way, those are still extra hoops to jump through and extra inconvenience, compared to me just logging into the web page, download an offline installer, and install it, regardless of what my son is doing on another PC with my (GOG or Steam) account.

I guess I need to educate myself what restrictions, if any, there are to using the same Steam account on several PCs, at the same time. I think at least running the same game on two or more different PCs using the same Steam account is blocked by default, but maybe it is possible to install and run different games... Either way, educating myself about those possible restrictions (and remembering them) is an extra inconvenience that I don't have with e.g. GOG offline installers.

I know this discussion has already gone beyond the scope that the OP intended, but just to remind that he was not complaining about lack of or delays in offline updates, but the lack of separate offline patches on many GOG games, meaning you have to redownload and reinstall the whole game with offline installers.

But as you suggested, maybe the best solution for that is using Galaxy with its autoupdate and delta-update features.