I still have the older version of Galaxy (1.2.57, not 2.0) and I know you can adjust the info file (in my case that's "goggame-1412601690" for Diablo) for each game to make Galaxy launch another program in the same folder as the game.
I'd like to make Galaxy launch The Hell 2 mod.

I've attempted changing it from this:

{
"buildId": "52336385680515400",
"clientId": "51903634292510995",
"gameId": "1412601690",
"language": "English",
"languages": [
"en-US"
],
"name": "Diablo",
"playTasks": [
{
"category": "launcher",
"isPrimary": true,
"languages": [
"*"
],
"name": "Diablo",
"path": "dx/DiabloLauncher.exe",
"type": "FileTask",
"workingDir": "dx"
},
{
"category": "game",
"isHidden": true,
"languages": [
"*"
],
"name": "Diablo (DX)",
"path": "dx/Diablo.exe",
"type": "FileTask",
"workingDir": "dx"
},
{
"category": "game",
"isHidden": true,
"languages": [
"*"
],
"name": "Hellfire",
"path": "hellfire/hellfire.exe",
"type": "FileTask",
"workingDir": "hellfire"
},
{
"category": "document",
"languages": [
"*"
],
"name": "Manual",
"path": "MANUAL.PDF",
"type": "FileTask"
},
{
"category": "document",
"languages": [
"*"
],
"link": "http://www.gog.com/support/diablo",
"name": "Support",
"type": "URLTask"
}
],
"rootGameId": "1412601690",
"version": 1
}

to this:

{
"buildId": "52336385680515400",
"clientId": "51903634292510995",
"gameId": "1412601690",
"language": "English",
"languages": [
"en-US"
],
"name": "Diablo",
"playTasks": [
{
"category": "launcher",
"isPrimary": true,
"languages": [
"*"
],
"name": "Diablo",
"path": "TH2.exe",
"type": "FileTask",

},
{
"category": "game",
"isHidden": true,
"languages": [
"*"
],
"name": "Diablo (DX)",
"path": "TH2.exe",
"type": "FileTask",

},
{
"category": "game",
"isHidden": true,
"languages": [
"*"
],
"name": "Hellfire",
"path": "TH2.exe",
"type": "FileTask",

},
{
"category": "document",
"languages": [
"*"
],
"name": "Manual",
"path": "MANUAL.PDF",
"type": "FileTask"
},
{
"category": "document",
"languages": [
"*"
],
"link": "http://www.gog.com/support/diablo",
"name": "Support",
"type": "URLTask"
}
],
"rootGameId": "1412601690",
"version": 1
}

However, Galaxy no longer recognized the game and I had to import the folder with Diablo and I was back at square 1.
Does anyone know what went wrong?

Since this has gone without a response for several days I'll offer the following as a possible solution. Which I'd have done earlier but it's been so long since I've done anything like what you were attempting I couldn't make sense of it in anything like a reasonable time so I could be sure what I had in mind would work.

Details for what I suggest can be found in the thread "Utmo's Stash."

Rename the mod loading executable to that of the hellfire executable thus GOG launches the software that loads the mod and rename the the hellfire executable to anything you like and have the mod loading software start the game.

You only have to add this ( for Belzebub-FR, change witch Belzebub.exe for the english version ) :


{
"category": "game",
"languages": [
"*"
],
"name": "Belzebub",
"path": "Belzebub-FR.exe",
"type": "FileTask"
},

This tweak work with all GOG games ( I launch Quake mods and Doom source port inside GOG Galaxy )

My thanks to both of you for your replies. :)

I barely check GOG's forum in favour of Magog and in the begining of October I got acess to GOG Galaxy 2, so I don't have a way to test the game on Galaxy 1.2, as version 2 replaces the older version.

Since I can't test anything, I can't see what solution would work; so I'd be reluctant to mark either reply as a solution I can't test, sorry. :(

Old post but i just got Diablo and the Hell2 mod and i want Gog to track my gametime, i got it to launch the Hell2 instead of the default Diablo just with the GOG launcher features but it is saying it can't sync cloud saves and it doesn't seem to have noticed i launched the game as when i exited the game it still said played 2 hours ago, any tips?