Troubleshooting
Most commonly on Macs, users may experience a blank screen on flash players. Symptoms may include being able to hear the video, but not see it. In the case that this is your problem, or maybe not, attempt the following steps to arrive at a possible solution. You will either need to alt-tab out of Garry's Mod or close it.
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Disable hardware acceleration.
While on the screen of the blank YouTube video that is not working correctly, right-click the area where the video would normally show up to bring up the Flash Player context menu. Select the Settings option from the menu.Unselect the option Enable hardware acceleration and click the Close button. Close out of the browser completely, relaunch and test YouTube videos.
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Attempt one or more of the steps from the following website
https://www.online-tech-tips.com/mac-os-x/fix-black-screen-youtube-and-flash-videos-not-playing/
Remember to restart Garry's Mod after attempting a fix. If the problem persists, please post a detailed description of what's happening either on the Cinema workshop page or the Facepunch thread.
After closing this window, you can reopen it again by pressing F1.