I am sure this has been asked, although unfortunately I could not find an answer.

If I download a game using the downloader to: default directory ... and then move the game to: somewhere else directory...

And then some time later, I notice there is an update in the library, does the GOG downloader only download the updated files, or does it download the entire game, and I have to do a reinstall?

I have some updates in the library, and wondering how this works. Thanks all!!

- keep the same name directory: . You can put the directory on another drive , but i suggest you copy the directory in the one where the downloader put the files. There's also an option to rename the directories with 2 batches (one for better visibility , another to rename them with the names used by the downloader).

see there : https://onedrive.live.com/?cid=201ecfa3dd196829&id=201ECFA3DD196829!165&authkey=!AChGfR7JnrYGTiE

of course, you need to edit the batches files to only include the games you own.

- but whatever you do any update won't updates any installer (that''s not possible)

GOG still has a bad beahiour to reuse some gog build number (that could lead to some issue if the installer you have in the directory has the same build number) . Sure it's way less frequent .

After that unless gog doesn't offer a patch to update your game , but rather offer only a full installer (happends when the update is too big , the game is so small in size it doesn't worth to push a patch , or when there were too many patches in the past)

take the patch, if you are playing the game currently, otherwise the full installer (if your download speed is correct and / or you don't have any quota)

Just copy the UPDATED file in your copied game folder.
It's the OLD school way of updating game.
Mostly games will work fine when you move them to other directory still there is chance that GOG downloader give you a message "Game NOT found".
I don't use GOG downloader instead I download game installer and patch them accordingly.

Thank you all for replying and helping out. Much appreciated!!