So I think I read this back when I played non-GOG version, but Hellfire doesn't support mutiplayer right?
But, somewhere or somebody leaked the trick for developers how to unlock multiplayer and locked content (cow level):
You have to create a .txt file, named:
and saving for Hellfire folders.
And that's all!
Still working GOG version too, so don't worry. But only playing Lan, or IPX Swapper program. IPX has inside GOG version too.
I hope you helping. :)