Sorry if this has been answered already; I did look in the FAQ as well as the forums but can't find a definitive answer.

If I buy the Diablo + Hellfire from GOG, can I just run/install only Diablo 1? Or is Hellfire "bundled/patched" into the main game? I have a buddy that swears by the Belzebub mod but I'm seeing that only runs on Diablo 1.
Thanks in advance!

I can't answer your question, but I can ask you for some more information that anybody who can directly help you will probably need. First, are you using Windows, MacOS or Linux, and which version? Second, have you installed Diablo yet and if so, have you tried to run it? If you have how did you try to start the program and what happened when you did? There are other questions that might need to be asked, but that's enough to get you started, once somebody who knows more than I do joins the conversation.

Hellfire and Diablo are installed as two separate games you can run either when ever you want.

Belzebub is it's own thing it's a recreation with some fan expansions and quests, a lot of Diablo 2 mechanics. It doesn't care what your base game is as it's not a mod that runs inside the original.

For Mac, Linux, Android, modern Windows, and more you can use DevilutionX to run the game with a bunch of bug fixes and enable various QoL like alt highlight, 60hz full resolution etc.