I don't own Diablo I on GOG, but I used to play it a lot. Both the original and Hellfire from a CD as well as Belzebub. Belzebub is definitely the best way to play Diablo I. It not only makes the game perform better and allow you to play in higher resolution, but it also adds many nice things. From QOL stuff like waypoints, crafting, better merchant, stash interface, classes have their own special skills which make their gameplay much more unique, better saves. In Diablo I you saved the actual state of the game, the game was linear and you weren't supposed to go back. The enemies didn't respawn. But here it works like in Diablo II. With waypoints going back to where you were is not a hassle and you can refarm the floors. The original game picked up for you few quests from the pool randomly. You had only few quests per gameplay basically. This mod streamlines every quest so they all appear so you actually complete the whole game. It also adds the missing content and quests as well. The group remade the quests with the data that was lurking in game files and they did fantastic job. If you didn't knew the quests were made by the modders you might think they were the work of the original team. The difficulty for normal difficulty is easier than original game, because you can refarm the floors. But the higher difficulties are actually harder. It's worth playing the original and Hellfire at least once to appreciate the mod more. If only they added Hellfire content in the mod it would be even more amazing.
Here are the problems I've encountered and the solution. TLDR: Just install original Doom III and DHEW3 source port. Follow the instructions in DHEW3 (copy the pk3s from the original game). Download Lost Mission from moddb (expansion from BFG). I've started from Doom III BFG, because it is newer and has additional expansion so I thought it might be better. It launches without problems, but in the game the mouse is not moving the gun. It seems it's impossible to bind mouse movement to "look up", "look down", "turn left", "turn right". I've read it's a console port and maybe that's the reason. I've tried various fixes I've found somewhere like deleting joy config files and renaming the exe. But no such workaround worked for me. I know that classic DOOM had such controls originally (moving the gun with arrow keys), but even DOOM I/II can be played with a mouse without a problem and it's just more comfortable. I've tried alternative engine RBDOOM, but it didn't help either. Nothing I've tried fixed the problem of not having a "free look". After not being able to play Doom III BFG with a mouse. I've tried original Doom III. It's considered a better version by the community, because it's not a console port and has better moddability. It comes with Resurrection of Evil expansion and the last patch - 1.3.1 which is exactly what we need if we want to play with mods. The Lost Mission expansion which was exclusive to Doom III BFG has been ported to this version as a mod which is available on moddb, so it seems like there is nothing lost if we aren't able to make BFG work like in my case. With the mod we have all 3 campaigns and the original has no content cut from it. Thanks to mods we can enhance the game so it looks and plays better than BFG. But it didn't come without it's own issues on my system: When I first launched the game it asked me for a CD key, but I couldn't do anything. Mouse didn't work and menu can't be controlled with a keyboard. I needed to edit the key manually outside the game. The files are base/doomkey for Doom III and base/xpkey for ROE. The keys are in our library on GOG under Doom III -> More -> Serial. I just inserted the serials on first line of these files. After launching the game next time there was no prompt for a key. The second problem is that even then I still couldn't use the mouse in the menu. It was stuck in upper left corner of the screen and didn't respond to any movement of the mouse. It looks like it's very common issue, because there are many posts on various forums, Steam etc. I've tried some of the fixes that people recommended but they didn't work. Finally I've tried from scratch with a source port that people recommended called DHEW3 and it fixed the problem with mouse stuck in the corner in menu and freelook in the game so finally it is playable. DHEW3 is the definite way to play the game. It apparently fixes many issues in original game. It actually works even on my stubborn Windows 10 system and is compatible with mods so there is no reason not to use it. It just works.
