So Hellfire from CD works fine with GOG's version. Heres how to do it. I did this under Windows 10. For original Diablo (Non GOG scroll down.)
1) Install Diablo. (GOG's install should run fine.)
2) Install Hellfire. (HD Voices are optional.) Make sure to not let Hellfire update via setup or register or anything.
(Setup should run if it fails to then copy the following files from the Hellfire CD to a new folder in your Diablo folder.) (Feel free to name the new folder to whatever you want.)
You will need:
storm.dll (If you have copied these files across rather than running setup.exe then skip to step 4.)
3) Go to C:\SIERRA\ and copy the entire HELLFIRE folder to your Diablo installation.
4) Download Hellfire Enchanced
If you don't do this then Hellfire will ask for the Diablo CD. It shouldn't change anything else other than making Hellfire more reliable under newer systems. Extract it to Hellfire's folder and replace anything it wants to.
The game should then run from hellfire.exe. You can then uninstall Hellfire and Sierra Utilities from Settings then Apps if on Windows 10 or Control Panel on others or 10, as uninstalling it through Sierra Utilities does nothing. If you just copied the files over then you can ignore this. [Optional] To get Hellfire running using GOG's wrapper (as discovered by Realpsojed):
1) Run Diablo Launcher from GOG, select Video Settings and setup your graphics.
Recommended setup is:
Display mode: Same as Desktop
Presentation: Windowed - integer scaling
2) Copy the ddraw.dll, dxcfg.exe and dxcfg.ini files from Diablo\dx into your Hellfire folder (replacing the files it wants to replace.)
Next time you run hellfire.exe it should work at a higher resolution though much like GOG's version you will get black bars down the side of widescreens. Any issues with the above feel free to ask here or PM me and i'll try to help you.
I did this and then i copied all files from my hellfire folder to my diablo folder.
I did overwrite all files.
Then i renamed diablo.exe to 1diablo.exe
Then i renamed hellfire.exe to diablo.exe
Now i can start diablo and hellfire using gog galaxy.
Run diablo using gog galaxy, then the launcher starts.
Select Diablo to start diablo.
Select Diablo classic to start hellfire.
I dont care about battle.net multiplayer.