DarkRP, TTT and Prop Hunt Gamemodes
Garry's Mod gets its strength from gamemodes. You choose which mode runs with the +gamemode parameter when starting the server, and you can run several GMod servers with different modes on the same machine.
DarkRP
The most popular roleplay gamemode. Install DarkRP from GitHub into the gamemodes folder and the darkrpmodification addon, where you define jobs and items, into addons. Jobs live in jobs.lua and shipments in shipments.lua.
Trouble in Terrorist Town (TTT)
Ships with the game and needs no extra install. Most TTT maps use Counter-Strike: Source content; download it to the server with SteamCMD (app 232330) and mount it with mount.cfg.
Prop Hunt and other modes
Modes like Prop Hunt, Murder, Deathrun and Jailbreak are added from the Workshop. Add the mode to your collection and change the +gamemode value in the launch command.