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You can learn more about game cache verification here Verify Team Fortress 2 and other mounted games, if any Press "Verify Integrity Of Game Cache" button.Select "Properties" from the dropdown menu.If that doesn't work for some reason, you can do it manually: Sometimes game files might get corrupted and a file verification is necessary. If when you start Garry's Mod the screen is black - your display settings might be wrong.
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