GunGame is a plugin created using the Source.Python server plugin for Source-engine games. As such, it requires Source.Python to be installed on your CS:S or CS:GO game server.
GunGame is a leveling mod where players must advance weapon-by-weapon. The first player to get through all weapons is the winner.
To install, simply download the current release (not currently available) and extract the contents into the main directory on your server.
Once you have installed GunGame on your server, simply add the following to your autoexec.cfg file:
sp plugin load gungame
After having loaded GunGame once, all of the configuration files will be created in your server's ../cfg/source-python/gungame/ directory.
Edit each of those files to your liking, then restart the server.