You should just buy it because it's only $10, however, if you really don't want to, then go here:
http://www.freshwap.net/forums/games/230098-uber-garrys-mod-11-non-steam-stand-alone-2009-a.html
or
http://www.final4ever.com/showthread.php?t=38684
For downloading the addons, go here:
http://freesteam.org/gmod/index.php
because garrysmod.org is only for people who have the game.