In case anyone is interested, I found away to run this with hellfire directly from the gog launcher with a couple of adjustments.
I'm not sure if this is actually needed for the process to work when using gog galaxy but before starting, I had the game set to NOT automatically update. An automatic update would more than likely break this!
1. Extract the files to the hellfire directory as you normally would
2. Rename your hellfire.exe file to something else (hellfiregame.exe for example)
3. Rename the loader.exe file to hellfire.exe
4. Open the PlugUlmo.ini with a text editor and make the following changes:
4.1 Change "exe_file=Diablo.exe" to "exe_file=hellfiregame.exe" (or whatever else you renamed your original hellfire.exe file to)
4.2 Change "stash_name_solo=" to "stash_name_solo=Hellfire_SP"
4.3 Change "stash_name_multi=" to "stash_name_multi=Hellfire_MP"
4.4 Save the changes to PlugUlmo.ini
5. Launch hellfire as you normally would
Steps 4.2 and 4.3 are only needed if you already had items stored in a single player or multi player stash. This process will still work if you perform these steps anyways.
This method may not be compatible with other mods which use a custom hellfire.exe file
These instructions assume you are starting with the unmodified original PlugUlmo.ini, If you had already changed it to run the mod in hellfire, then step 4.1 would become: Change "exe_file=Hellfire.exe" to "exe_file=hellfiregame.exe" (or whatever else you renamed your original hellfire.exe file to)
This method might also work with the base Diablo game if you replace all mentions of hellfire with diablo but I have not tested it so I can't say for sure.