How to Play as TF2 Characters in GMOD?

Many of you have seen this video:

In the video you can see TF2 Characters been played in various HL2 maps.
First i thought it was greenscreened, but i was wrong.

I tried searching etc.
Does anyone know anything about this?

TF2 gamemode I assume

Obviously the TF2 gamemode.
Shit rate me clocks.