Suicide Barrels is a Garry's Mod gamemode that's been rewritten for Sandbox as an example. The code for this gamemode is on GitHub so you can get an idea of what it's like to make gamemodes in Sandbox.
This gamemode now uses the shooterbase but the original code in the repo shows how you can make a gamemode from scratch without any bases.
For testing I coded dumb bots, they actually manage to win rounds even though they just run around in circles and follow people.