To link an EXE file I must first add a game to my library. To do this, I must find it using GOG's search, which as far as I know links to IGDB. So, if a game is not in IGDB's database, it is impossible for me to add it to my library and launch it from GOG. Unless I'm missing something, anyway. This would mean I need to sigin up for IGDB, submit a database entry and go through whatever that process entails just so I can add a locally installed game to my library, which is frankly rediculous.

The reason I ask is because I purchased Humble Monthly's November bundle which includes an original game, Operator. I cannot find this using GOG nor in IGDB's database. Is there no way to get around this and add it manually? If not, there really should be a way to add a dummy entry with information the user fills out themselves.

Alright, I tested this today.
-The Gog library is indeed linked to the IGDB database.
-I added a new entry there this morning, and it has been accepted and released the same day.
-Galaxy 2.0 automatically picked up on this (so it is a live link) and I was able to add the game (Worlds: Histroy Simulator) manually to my library.

Yes, the process is a bit lengthy, sure, but the IGDB interface is decent and offers much for details. Adding a game there not only adds it to your personal library but offers it to anyone else: we all help each other out this way.
It'd be great to do this form-filling from within Galaxy, and would't be too difficult I guess, given the browser based format. But I'm not sure Gog will want to go that way, IGDB being owned by Twitch now. But, they very well might.

Edit: One big flaw I am noticing with this set-up is how GOG's database differs from the IGDB entries, a.e. a game available on gog but entered manually will show up with different data/categorization.

This is an issue for several games available on itch.io. Try searching for 'Mini Doom 2' and there are no results. The same could be said for any number of titles one could add (doesn't help that itch.io integration doesn't appear in the community integrations search results in the Galaxy preferences but this affects games other than just itch.io).

Ideally Galaxy would allow the user to fill in the metadata and link to the executable themselves, which IIRC Steam allows for. That way it would indeed be an all-in-one launcher that could replace entirely alternatives.

I think this is honestly a good idea to add instead of waiting because it would be especially useful for standalone mods like Open Fortress, a conversion mod for Team Fortress 2.

Hey people from the past. This still an issue because GOG sometimes doesn't pick up games that are on IGDB, for example super mario bros 1,2 and 3 can't be found. I do all sorts of listings on GOG galaxy despite its bugs and it would have been easy and convenient to allow us to add custom game to YOUR library, edit the title and maybe optionally the picture/description.

edit: there is a workaround. A way to "manually add any game" so to speak. Add whatever existing game and edit its title, year and icon. Voila.