go to garrysmod.org (Not all of the girl stuff)
go to steam.com here are some links related to Gmod to help you: Steam: http://store.steampowered.com/ Download stuff to Gmod here: http://www.garrysmod.org/downloads/ About Gmod: http://www.garrysmod.com/about/ Gmod fourums: http://www.facepunchstudios.com/
piggymoo.com has all of stargate, but I'm not sure what gmod is.
gmod 9, yes gmod 10-no
Download the playermodel addon from the workshop
You need to download Gmod 9.0.4c when you start there will be 3 choose unmark the Gmod 9.0.4b and your set to go and play gmod 9 online.
I would recommend garrysmod.org
download them on garrysmod.org, or port them mingebag
download them from garrysmod.org
No because Garrynewman has not made l4d contact for gmod but if you type gmod in google you will find Garry's mod.org and click then type l4d and it shows l4d stuff
To download gmod for free, you must first find a website that offers the software for free. A simple search through the search engines should bring up quite a number of options for you to choose from.
It all depends on your internet connection speed. Mine took about an hour to download, while my friend's took four hours. Right before Gmod starts downloading, it will tell you how long it will take.
Steam is your best bet, it provides regular updates too.