If it's an SVN version, then it shouldn't be a .zip, you'd need a program called Tortise SVN to download the SVN version of Spacebuild 3, and you'd need to set the download path to your steam/steamapps/<USERNAME>/garrysmod/garrysmod/addons directory
Most SVN's that i know of are in 'Addon format' with how they have the info.txt and stuff, so the download path should be set to your addons folder