This demo includes the first three maps of the mod, and has recently been updated with fixes to the materials and models.
From the mod's full description:
This mod is a short 3 chapter story about one of Gordon's side missions where he trys to escape the combine. He ends up in an accident and is left to go through a warehouse that you must fight through to escape. In this story you will fight zombies and combine only. So get ready because this mod will take you through a quick side story that is somewhere in city 17.