One way Walls.

I saw on a DarkRP server the other day a guy made a wall that you could see out one side but not the other. Does anyone know how to do this?

You use material tool, select the white mat.

Could you be more specific?
Was it a prop? Which prop?
Was it a colorable wall?

I will get the preop name and mat name once i play gmod in a bit

The prop you are referring to is a standard half life 2 model. Shop window security barrier I believe. With it’s standard texture being metal bars and mesh. You can find it under Useful Construction Props.
When materialized however it does essentially become a 1 way window. Never seen any other prop model do this. Weird texture coordinates maybe?