If you want to use a Steam overlay or specific controllers with a non-Steam version, you can add it as a shortcut: Open Steam and go to the button at the bottom left and select Add a Non-Steam Game Browse to the Strogino GMod launcher and add it. 4. Mounting CSS Content
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Historically, it was a popular way to get GMod pre-loaded with CS:Source textures, which are normally required to avoid "missing texture" errors in the official version. If you want to use a Steam overlay