1) Download & Install TortoiseSVN - http://tortoisesvn.net/downloads.html (http://tortoisesvn.net/downloads.html) (Either 32bit, or 64bit)
2) Once you've successfully installed TortoiseSVN, go to your '/steam/steamapps/youracc/garrysmod/garrysmod/addons/' folder.
3) Create a folder, named: hgnstalker (Name doesn't really matter).
4) Right click that folder, and hit 'SVN Checkout' - Now copy paste the SVN url from Dark Dolina:
https://hgnstalker.svn.sourceforge.net/svnroot/hgnstalker/ (https://hgnstalker.svn.sourceforge.net/svnroot/hgnstalker/) and hit OK.
5) Wait till it's done, and ENJOY.
PS. If you don't have EP2 you might be missing some models, not such an issue.