I mostly just know Garry's Mod from weird YouTube videos. What's it like as an actual game?
It's very dependent on modders to have a replayable experience. The basic mechanics used in sandbox, with no modification are basic FPS movement, very basic gunplay, entity ("props) creation, ragdoll manipulation, and basic NPCs.
Modifications are the main idea of the game, and they're relatively easy to create as the scripting language used is gLua, a very slightly modified version of Lua. There's a ridiculous variety of gamemodes, but the most popular are;
Various types of roleplay - Dark (basically cops and robbers with the occasional homeless person), military, star wars... harambe
Trouble in Terroist Town - Very similiar to mafia, albeit with more action based gameplay.
Murder - Simplified version of mafia, albeit again with more action based gameplay.
Prop Hunt - Hide and Seek where you disguise yourself as an object before hiding
Unfortunately the commnity is generally awful tweens and teens, but the few good people have kept me coming back for so far about 700 hours.