This is a technical question. I am wondering if there is a way for a third-party program to query where GOG games are installed.

I have a use-case for this, which is basically . ZDoom is a source port that can run Doom, Heretic, Hexen, Strife, and [url=http://zdoom.org/wiki/IWAD]more and for user's convenience it automatically detect the game data files (called IWADs in Doom's jargon) from their Steam install folders so that people don't have to look for them and manually copy the files into ZDoom's own install folder. Since the Doom games are now also sold on GOG, and will hopefully be followed by Heretic, Hexen, and Strife, I would like to provide the same convenience for people who purchased these games on GOG as for those who purchased it on Steam.

Looking through my registry, I don't see anything that looks like a GOG install directory (unless it's somehow encoded in that mysterious HKEY_CURRENT_USER\Software\GOG.com\Secret or HKEY_CURRENT_USER\Software\GOG.com\Token values). I do believe however it's kept track of somehow because the first time I installed a game from GOG I changed the install parent folder it wanted by default (C:\GOG Games IIRC) and after that every other game took the same parent folder I chose the first time instead of suggesting C:\GOG Games again.

The installer saves its last used path in HKLM\Software\Wow6432Node\GOG.com (for 64-bits) and HKLM\Software\GOG.com (for 32-bits). Doom should store its path either at the above\GOGDOOM something, or at the above\Games\{id}. Both should be in the key {path}. Do a search for doom in the registry, under those keys, and you should be able to find it.

A bit off topic, but are we saying many people install games to many locations, or is this a Galaxy thing?

I always install GOG games to a folder c:\gog\<game>, i.e. outside the program files, as it avoids the whole UAC issue and keeps them nice and organised.

As for OP, I would suggest having a "add your own path" option, maybe it can default to that install, but likelihood is there would be more than one install (i.e. some might be modded etc.).

Thanks! I basically do the same thing, though it's more like D:\Games for me. Oh, there are already ways to add more search paths for the game data files, but this feature represents something done automatically and transparently so as not to require the user to do anything.

That's nice. Personally I would prefer to be in control however and navigate to a folder.