This is the engine GMod 9 runs on. It is available for free in your Steam Library. Installation Guide
Right-click in your Library and select Properties . skachat garris mod 9 no steam
Avoid "no-Steam" standalone cracks found on unofficial sites, as these often contain malware or are incompatible with modern systems. Using the free Source SDK Base 2006 is the only safe way to play GMod 9 today. This is the engine GMod 9 runs on