This map is pretty ambitious and thus there will be JANK, here are potential solutions to your woes:

Enable color correction and set shadows to high in advanced video settings, also disable HDR and bloom.
Also don't forget to install required addons.

Change your playermodel.
The way hud is done in this map is by straight up welding it to your playermodel's face, aka Forward bone specifically. If your playermodel doesn't have it - there's nothing for hud to attach to and so it doesn't.
(Usually combine soldiers don't have forward bones for some reason)

Restart the map. I tried fixing this but it seems to be a little gmod quirk. This only happens when you boot the map up for the first time after launching gmod.

Go to your graphics settings and set texture quality to whatever it was before. In console, enable texture filtering back on by typing mat_filtertextures 1
I used lua to change texture quality and texture filtering as a means to save time back when this map was being created for a friend.
Texture quality and texture filtering only get changed when you load this map.