First Go on to google, and type in Garry's Mod 9. Click on the First Option! When you click on it, it should look like this!

Scroll down a little bit and you'll find a box that looks like this Click the Red Button and download the File!, Once that is complete go to your downloads folder and find the folder that looks like this

Now Click the Gmod_9_0_4 Folder open it up and it should look like this Click Next, it will then take you to this, if you have enough space (I would Recommend 300 Mbs But if you can get to the requirnment then its ok) Next after clicking install wait for the files to download which shouldn't take very long.

Go to your "Programs files (x86)" file and click your steam file, then scroll down to steam apps and click it, after going into steam apps, look for the source mods file, click that, then click the Gmod9 File, scroll down until you find the gameinfo file, click that and a screen should pop up looking like this Near the bottom You should see a pair of numbers, (220) click them and delete the 20 out of 220, and change it to 215. after that you can exit the file.

Now if you were to go back to your library, you should find your games, and Garry's Mod 9 if you try to play it will tell you, you need SDK source 2006 so if you have enough space go and download it, its only 2gbs. After that, You should be able to play!

Thx for clicking plz rate if it worked for you! If you had any trouble plz tell me in the comments.