At http://www.garrysmod.org
You can download numerous drivable vehicles, I suggest getting a mod called "Vmod" which contains many vehicles to drive. IT can be found on the website or by Googling it.
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
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/
go to garrysmod.org (Not all of the girl 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.