People seem to have alot of problems making it work. I however didn't. You just have to….
1.download the full doom 2 from kazaa
2.download zdoom and gbdoom from this site
3.put it all in doom2 folder
4.you may have to open the zip and edit z-gbd2.bat and remove the phrase “D:\games\doom2\” from the beginning so it just read “zdoom.exe -file zdoom.wad gbdemo.wad gbgfx.wad colmaps.wad -deh gbd2.deh”.

now your done then just click on the gbd2 file

Before you point it out I know this has already been posted before.