Grey Source is a modified version of the Half-Life 1 engine (aka Goldsource) developed by PHOENIX.
This is version DEM-alpha-d1-v1.0 [build 2005] of Half-Life: Zombies Ate My Neighbours.
You can use software such as PowerISO to mount the ISO to a virtual drive. Or, you can extract the ISO to anywhere of your liking.
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Demo
Official
Release