The GZDoom FAQ
A: GZDoom is a Doom source port based on ZDoom. It features an OpenGL renderer and lots of new features, among them:
It also contains many bug fixes not present in the most recent ZDoom versions.
It was originally based on the 2.0.96x community build but it has integrated all new features and bug fixes from 2.0.98.
Sure. GZDoom still contains the complete software renderer. The only limitations are that you can't use 3D floors, dynamic lights, Q2 style skyboxes and True color textures. In effect you get ZDoom with all the bug fixes and non-renderer specific new features. That includes complete .96x DECORATE support including inventory items and weapons.
No. As long as you don't use any GL-specific features your WAD will run perfectly with GZDoom's software renderer and since that is the same as ZDoom's it should run on any system that supports ZDoom. As of version 0.9.28 it is possible to disable the entire OpenGL subsystem in the menu so that the engine behaves as if it wasn't there at all.