Garry's Mod is a physics mod on Steam.
Here's the tut on how to get Garry's Mod.
Things you'll need:
A PC that can play it.
A credit, debit or paypal. Pre paid one's are fine to use as well.
£10 or $10 on you're credit, debit or paypal account.
A Steam account and the Steam Client.
1: Go to http://store.steampowered.com/
2: Make a account on any username you want as long as it's not taken.
3: Download and install the Steam client.
4: Login to you're account
5: Go to the Store tab and type in the search, "Garry's Mod 10."
6: Go with CS:S and Garry's Mod option as CS:S is basically required as all the content is CS:S and will be errors.
7: Checkout and go input your credit card numbers and all that, you really know what to do.
8: After that, go to the Library tab, you should have two games that you can install, Counter Strike: Source and Garry's Mod.
9: Click on CS:S and click install and choose the options and install.
10: Do the same for Garry's Mod and then wait for both of them to download. The games are a few Gigabytes big so it'll be a while.
11: After they've finished downloading, go to Garry's Mod and click play, Enjoy!
Tut by xVENUSx
Game by Garry Newman