GoldenEye: Source requires Source SDK Base 2007, which you can download from Steam. For more information, check the installation guide. The current latest version of the game is 5.0.6.
Direct Downloads
5.0.6 Full Installer
Download from ModDB
Download from GameStand
Download via BitTorrent
For the full installer, it is recommended and preferred to use the torrent links below.
This requires a BitTorrent client such as qBittorrent.