This requires Counter-Strike Source to be brought for your Steam account
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Install Counter-Strike Source and Source SDK Base 2007.
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In the \steamapps\common\Source SDK Base 2007 folder create a new folder called cstrike.
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Then download the GCFscape Installer and install it. I used the "GCFScape v1.8.5 Full" version.
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Open GCFscape and click on File > Open and go to steamapps/common/Counter-Strike Source/cstrike and find the cstrike_pak_dir.vpk file and then press Open.