Let's do this step by step.
1.)you own doom2 and have it installed on your computer. Out of curiosity, did you buy your copy of doom2 or get it elsewhere?
2.)you downloaded both zdoom.zip and gbdoom.zip and exctracted them both into your doom2 folder.
3.)You right clicked on the z-gbd2.bat file and removed D:\games\doom2 from behind zdoom.exe and saved it.
4.)instead of running zdoom, you double clicked on z-gbd2.bat.