To use the Kinect in Garry's Mod you need a few things.

A Kinect

It doesn't have to be the Windows version of the Kinect - but you need one. You can't use your webcam, or some alternative. It needs to be a Kinect.

The Xbox 360 version of the Kinect uses a different connector, since the device needs a power supply. For older Xbox 360 models (non-slim) and, in our case, computers, you will need to buy a Kinect USB/power adapter.

Third-party binary modules can extend support for more versions of the Kinect. Namely, gmcl_rekinect allows you to use the Xbox One version of the Kinect. You may also need special USB/power adapters to use these Kinects with your PC as well.

Kinect for Windows Runtime v1.8

The runtime files are available here. You might already have this installed if you've used your Kinect on your PC before. But you'd probably remember downloading it.

Kinect for Windows SDK v1.8

(These files may not be required but it may fix the Kinect features for you if they aren't working, as it has for some) The SDK files are available here. You might already have this installed if you've used your Kinect on your PC before (same as the Runtime). But you'd probably remember downloading it.

More Room

It works best when the camera can see the whole of your body, from your head to your feet. So make sure your camera is set up properly so you can stand far enough away.

How to play

Go ingame, spawn a ragdoll, hold c (or whatever other key you may have your context menu bound to) and right click on it. In the menu choose "Control With Motion Sensor"

If the kinect option isn't there - then GMod can't detect your kinect for some reason.

Now stand infront of your kinect and the ragdoll should start to copy you.

Troubleshooting

If you join the game and it shows the kinect options but they don't seem to do anything, then first try rebooting.

If that doesn't work, and you have an Xbox kinect - then you need to install the Windows Kinect SDK 1.8 too. Give that a go.

Recording

If you download the Kinect for Windows Developer Toolkit v1.8 it includes a program called Kinect Studio which can be used to record your kinect input to a file - and play it back. This is useful in making movies by yourself, where you want to act something out and walk around recording it at the same time.

Kinect for Xbox One

Garry's Mod does not officially support the Xbox One Kinect. However, third-party binary modules like gmcl_rekinect can be used to get the Xbox One Kinect working in Garry's Mod.