This tutorial will walk you through setting up hammer to work with Goldeneye: Source maps on the Half-Life 2: Deathmatch enngine.
If you're a developer for GES, you will need to setup a sandbox, meaning a folder or partition that you are designating to store you development source files temporarily. In this example I will use C:\Sandbox as my 'sandbox.' Using SVN, 'checkout' the map section of the repository into C:\Sandbox\maps if your only working on goldeneye files, or if you might be involved in other mods as well, it is always good to section it off, C:\Sandbox\goldeneye\maps
For your convienance, the code is setup with a 'test' that will automatically check a few of these things for you and print a debugging message. The command for this is test_mapbuild.