In your Steam Library, click the dropdown menu above the search bar and ensure is checked. Search for Source SDK Base 2006 and install it. Set Launch Options:

In the box, type: -game "C:\Program Files (x86)\Steam\steamapps\sourcemods\gmod9" (Ensure the path matches your actual installation folder). Edit GameInfo (If Needed): Navigate to the gmod9 folder in your sourcemods directory. Open gameinfo.txt with Notepad.

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. It is available for free in your Steam Library. Installation Guide