Brilliant game. Had it since it first came out but dont tend to go on it now. Theres a huge number of things you can do on it. For example on the basic mode that ships with the game you can build anything from all the props that come with the source games. For example you can add wings and thrusters to the airboat thats in half life 2 and fly around in it.
Then theres the extra mods you can get for it that automatically download when you go on a server. Played left 4 dead? Yeah that whole zombie idea started on garrys mod. You have a team of players that start on a map. Everyones a survivor bar one guy that gets turned into a zombie. Its his job to break into the house/stronghold and kill someone. When that person gets killed they get added to the zombie team so theres now 2 zombies on the map and so on until the final survivors dead. Theresalso different classes that unlock the more survivors you kill such as fast zombies that can leap and climb walls and heavy zombies that can chuck headcrabs

As you can imagine it gets pretty hectic towards the end.
Then theres the spacebuild servers where you start on a planet and the idea is to build a spaceship and explore the solar system. A mod called life support is used on this so you have to build a fully functioning life support system including energy production. Oxygen scrubbers, coolant etc if your to survive out the atmosphere. Inevitebly this leads to space battles over territory etc.
Then theres the rp servers where you all fill the role of a particular character in a town/army base/spaceship. So this ranges from janitors and cooks to mayors and cops (both of which have to be voted in by the server which is what the op was on about) you also have the criminal elements producing drugs and bombs etc which the cops have to keep in check and by arresting criminals and putting them in jail. All very fun
If you want me to explain more just ask but well worth the price