A quick update: some time ago I've asked a professional artist to create GUI textures for Xenos. He has finished this work, and now Xenos looks prettier than ever! I'm updating the demo with new textures now, so you can check it out. Or just look at these pictures:
The new demo also has a couple of bugs fixed: objects are no longer usable from behind a wall, and container interface auto-closes when you move away.
Good progress on the GUI! Are you using Unity's default GUI system or one of the addons from the asset store?
I'm using my own wrapper around built-in GUI. See my thoughts on Unity GUI systems here: Chaoscultgames.com