Garry's Mod is a Half-Life 2 Modification
In Garry's Mod, the simple answer to this question is: Anything! Create a comic strip, make a crazy contraption, set up a huge monster fight, build a fort online with your friends, the possibilities are endless. The creativity of GM users never runs out and new innovations are seen almost daily. Here's a quick rundown of just how much freedom you have in Garry's Mod:
*Create any object or character ragdoll that exists in Half-Life 2 or Counter-Strike Source, then arrange or pose them as you please with a super-powerful "physics gun".
*Change the facial expression and eye position on a ragdoll to anything you please.
*Easily attach objects to each other in numerous ways, from a simple weld or rope to springy elastics, ball-and-socket joints, and pulley systems.
*Make any kind of contraption you want with keypad-controlled thrusters, rolling wheels of various sizes and types, and balloons to lift things up or weigh them down.
*Change the color and transparency of any object, or apply a Material to make something look like flowing water, stained glass, swirling energy fields, and a whole lot more.
*Unlimited levels of Undo! No worries if you've made a mistake; take back ballsockets, wheels, and everything in between with a single keypress.
*Create fully functional NPCs from Half-Life 2 and arm them with a variety of weapons to make your own massive Combine vs. Rebels vs. Antlions battle (or whatever else you want).
*Use the built-in Camera tool to take pictures of your work without the HUD, even adding a Depth-of-Field effect to blur the background or using the Bloom and Colour Mod effects to add vibrancy or drama.
*To add visual interest, paint an object with decals like bullet holes and blood spatters, or set anything on fire for the pyromaniac in all of us.
*All of this works online, too! Create or join a server for co-op construction or just a crazy game with your friends. You can even play through the chapters of Half-Life 2 online, with a few other players at your side and enemy health modifiers for increased challenge.
*There's even stuff for map-builders: Set up a complicated scene in Garry's Mod, then use the "Export to Hammer" feature and all of your objects will be handily relocated to the Hammer editor.
*Once you have had fun with the things you already have you can create mods for Garry's mod and add custom models in!
*Create your own weapon!
*Create your own Game-mode!