I have a game that I recently got on GOG as well as having it on another platform. Now I wish to put the already installed game in Galaxy instead of the other launcher, but obviously it is missing the Galaxy manifests and other stuff so it doesn't detect it as it's own.
If I could download the relevant files separately from my library and put them into the install, I could then run an update/repair to only download what is necessary to synch it to gog, instead of the entire install, to conserve GOGs bandwidth.
Is that possible?

Edit:
Yes it is - sort of. Start by moving the game into your gog library location (not required but better for management) and rename it, perhaps appending something to it. Then start the install through Galaxy and watch the contents of the folder it creates. When the files prefixed with "goggame-" appear (should be pretty quick), pause (not stop) the download. Then move the file ending in ".info.download" to the folder containing the actual installation, take note of the folder name Galaxy created and cancel the install (this will delete the folder and its contents).
Sadly, that file will be empty, therefore go to another of your installed games, find the respective .info file and open it in a text editor (there may be several, if there are addons installed, find the one that is not an addon in that case - they have descriptions inside so you know what the game is, or simply choose another game that has no addons installed). Copy it's contents into the ".info.download" file you just moved and remove the ".download" at the end of its filename. Now, take note of the number behind the "goggame-" prefix of the file: this is the game ID in the GOG database by which it will find the game. Within the file, replace every instance of number behind a "gameId": and "rootGameId": tag with the one you just noted. Replace the "name": instances with the name of the game and the "file": with the .exe the game (probably) will be. Likely these two aren't necessary, but I did not try without. Possibly just removing the ".dowload" from the end of the empty file even suffices.
Now rename the folder to what the install had in mind and use the "Search for GOG games", pointing it at the folder just renamed. If all went well, it will detect the game and start scanning files, then download anything that doesn't match it's checksums. This will also include the .info file you just edited, so don't worry about it's remaining content: it gets the proper content in the end. In my case, It ended up downloading only 3.6MB for the entire TR:UW install that would otherwise have been over 6GB. And it kept its settings and found the savegames from before the migration just fine. :)