http://www.doomworld.com/gbd2/develop.htm

Here's a link to a walkthrough and cheats I created at the old site for GB Doom.

Also Check out The Setup Page there to view settings for ZDooM that make the game play the best, and a complete guide to installation… *note* IGNORE THE NOTES ON DOWNLOADING BOOM AND THE COMMAND LINE FOR GETTING IT TO RUN ON THAT PAGE