This is a gamemode called Underdone 13. It was an RPG originally created by the Shell Shocked Gaming and Tower Yard Entertainment Team for Garry's Mod 12. It has now been remade and fixed to work on Garry's Mod 13 by The Commander.
These guys are very hard to take down and will even throw grenades, launch M203 rounds and even kick you if you are close enough.
In the distance a sniper can also be seen.