GOG provides for users to be able to download installers directly without using GOG Galaxy. So I suggest adding a parser or modifying the GOG parser to allow for parsing GOG games when GOG Galaxy is not installed.
As far as I can tell, all GOG games installed via their normal installers (possibly when installed via Galaxy as well) add a key to the registry on Windows (HKLM\SOFTWARE\WOW6432Node\GOG.com\Games) with all the information required to identify the game, it's path and other information.
I have a working test version in my fork I am not putting it as a pull request because I'm not confident my code is suitable for merging but I have successfully tested it on several titles.
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