@Woolie Wool recently posted a few screens showing off his GZDoom settings in the Post Your Doom Picture (Part 2) thread and gave a few tips on how to make yours look that way.
Woolie's Settings:
Basically what I'm doing is:
Resolution scale: 640x400, actual resolution 3840x2160 @ 70 Hz
cl_capfps 1
vid_vsync 1
NO anti-aliasing or texture filtering
No mipmaps
Distance fog: vanilla
Software-style banding enabled
Ambient occlusion set to maximum
Tonemap setting: palette
It