Well I've decided to jump on a bandwagon extremely late and got myself Garry's Mod for a whopping $2.49
I don't have Half Life 2, though, so this should be interesting...
The game comes with a handful of stuff from Half-Life 2 (since most Valve games reuse assets, it's easy to grab and copy them), but you may be missing quite a bit. Garry's Mod did start as a purely Half-Life 2 thing, after all.
Regardless, I find that the biggest draw is actually the multiplayer. Trouble In Terrorist Town is an especially popular one; try to go for the Crowbar Gaming server.
Is it possible to just fill in the missing assets with fan-made assets?
I really don't feel like buying a game I'll never play just to add assets to Garry's Mod.
Depends on what you want to add. There's a lot of mods out there (check the Steam Workshop for the game for everything that's definitely GMod 13 compatible, and it downloads the stuff into the game automatically for you when you start it up anyway instead of making you shuffle files around and breaking the game yourself), some of which are specifically out to add things like the Counter-Strike Source weapons or assets from various games. But I can't guarantee that you'll be running smoothly without Half-Life 2. One of the biggest problems, especially in multiplayer, is that servers will only download the model for something instead of the texture and it'll come out with a texture of purple and black blocks if it can't find something in the existing Steam files. Usually it can be mitigated by redownloading the specific stuff (though good luck if it's something added while joining multiplayer and you can't find the assets online), but sometimes it just plain expects you to have the right content.
I do know that there's a TTT pack on the Workshop in case you're missing Counter-Strike, so you can play what's literally the most popular multiplayer mode on there (as in player numbers of several thousand minimum at any given time).