[All comments go together as one, read from top first]

It's a great game actually, probably might want to play with friends because alone, you won't have much of a clue what to do. It's going to be a pretty big game file, because most people download mods/addons for it all over.
The standalone game comes with probably 2-5 maps to roam around on, not small, but also not that huge. You will have some NPCs and props available to spawn, so you can fight them, or watch them fight each other, make buildings with spawnable walls and such so you can create your own scene, etc. [There is no objective or mission in Gmod, because it's a sandbox game, you are given everything, and you have to create your own gameplay]. There are also ragdolls/puppets you can spawn and pose the body parts yourself, in case you want to create a stop motion movie or just have fun messing with dolls in funny scenes.

To go with addons, the workshop has a ton of things available. There are actual gamemmodes you can play that others create (but to play them, you might need some mods attached to it, so be sure you read info for addon requirements). Maps from tiny for pose testing, to huge for war-scaled NPC battles/ your own game made.