I need to free up some space on my SSD and unfortunately GOG galaxy doesn't have a move function.
Now just coping a game to my HDD I understand might cause issues. But the things is: I don't want to play PoE2 or any games I might "move" to my HDD in the future from the HDD. Just keep it there, so that I don't have to download it all over again, when wanting to replay it then copy it back to my SSD. That way all registry entries should be the same.

Basically:
1. Copy+paste game from SSD to HDD via explorer.
2. "Manually" delete it from SSD instead of uninstalling it.
3. When wanting to play it again copy it back from HDD to SDD in the same folder as before.

Would that work or would that break the installation and force me to uninstall and reinstall again anyway?
And how can I make sure GOG Galaxy doesn't just re-download it in an attempt to "fix" what it thinks is a broken installation?

Can't say anything in regards to the Galaxy-related questions, but temporarily moving game installations to somewhere else and moving them back again to the original location or path should work just fine.
Slightly different scenario (migration from Win7 to Win10) but I did exactly that a while back with The Witcher 3 and there was no problem whatsoever. Had to backup and restore the config and save files in their respective locations on the Win10 machine as well but that won't be necessary in your case. Can't hurt to back up those too, just in case something goes awry, especially if you have Galaxy possibly intervening in some way or another.

For longer periods of "outsourcing" game installations this way I'd recommend to uninstall and do a clean reinstall with the offline installers instead.

Yes this is possible, just copy the files to the drive for storage, when you want to put the game back click the install button on Galaxy, then pause the installation and copy the files of have stored into the directory that Galaxy created and then continue the download, Galaxy will see the files then verify them and complete the installation. I've done this before with multiple clients.

Another option you could do is Download the offline installer and save that to the storage drive, then when you are ready to play again use that to install the game, then have Galaxy scan your folder to find it.

Yes I probably should have DLed the offline installers to my HDD, but doing so now would kinda defeat the purpose of not having to redownload later.

Thx for the heads up about the rest.

Well, it is “possible”. I always recommend not doing so though. When something is installed it may install other elements such as cap extensions, registry entries and such like. So it may work, and for this one might be ok. I would highly recommend in future though you download all your offline backup installers for numerous reasons such as smaller storage requirements (they are compressed), they contain the full package needed to install, and they are more portable. Also, and most importantly this is the reason for shopping here, drm free!

the biggest drawback with offline installers is the whole new version thing though...... especially with those bigger files, of course if you have a working version you have a working version but still the new version might be of interest

Cleanest and easiest way. Copy the game somewhere else, then uninstall it via Galaxy.

If you want to play the game again, copy it back and use "Scan and Import" feature in Galaxy.

Yes, the whole download thing is an issue, see:
https://www.gog.com/forum/general/provide_a_full_and_complete_changelogged_download_system

However, the one single reason to buy here is drm free. If you do not keep the installers yourself, then that point no longer exists, you are relying on a third person holding and granting you access to your product.

And of course:
https://www.gog.com/wishlist/site/more_detail_on_offline_installer_pages
;)

Even better, import them from where you moved them and only move back to SSD the games for which the improvement in load times makes a real difference.

Yeah, I only promote my own :o). Have added your wish list to my thread.

well, to be honest, the single reason to buy here is to have acces to older games that work straight out of the box for me ;)
sometimes that is because of no drm attachements present, but sometimes that is not