Read the Description for how to Install The Citizen
How To Install
To install download the zip file and than go to steam. Once on steam right click and press Properties, than go to Local Files and than Browse Local Files. Once done that go into the folder "garrysmod" than find "addons" Drag the folder into there (Not the Zip File!) Once done that boot it up and find the map thecitizen_part1 in Other and than bam you got The Citizen up and running on Garry's Mod.