First off, there is a search to the top right. Second, to find the Downloads section you need to go to the main forum page and scroll down till you see "Freesteam Downloads" then in that section is "GCFs/NCFs ( Game Files )". You're going to need to look in the "Half Life 2 / Source Engine GCF's and NCF's" which is in the "GCFs/NCFs ( Game Files )," Garry's Mod is the 6th link from the top (at this moment).
To get addon's for GMod click the link at the top of any of the pages here that say's "Garry's Mod Downloads."
Hope that helps.