Zombie Overrun is a gamemode for Gmod with 2 teams (Survivors vs. Zombies)
Survive the zombie outbreak until rescue comes
But if you lose your health you'll have to hope for a teammate to come by to help you up, else your a zombie.
Find and destroy the generators for the machine that makes zombies
Search and buy weapons
Make a barricade
Block some doors and/or windows
Help teammates lying on the ground
Make a boobytrap
Chop a head of
Kill the survivors by hitting them or throwing stuff (Rocks, knives, a television, a teddybear etc.)
Hide in dark spots to wait for humans to kill
And more bloody stuff...
No Zombie team anymore