Instructions
Press the "Ctrl," "Shift" and "Esc" keys simultaneously. This opens the task manager directly.
Click the "Processes" tab. Click the check mark box beside "Show processes from all users." Wait a moment for the list to refresh.
Right-click "hl2.exe," which is the process "Garry's Mod" runs under; select "Set Affinity." A dialog box will appear.
Click the check mark box beside the unchecked core. You should have both boxes checked. Click "OK" and restart "Garry's Mod."