Yes I mean Half-Life 2, and not G-mod. This was made using Face Poser and the Hammer level editor, both of which are included in the Source SDK, downloaded under the Tools tab of Steam.
This was actually the first video I've made in a very long time to use Windows Movie Maker. I was home from college for a while, and spent my time making this. It took a total of about 2 days to put this together; basically a night of mapmaking and debugging, and a short afternoon of editing.