You need to go to this website and select your operating system and select 'For Other Browsers' and select download. Make sure you don't install any other software that comes with it, beware of what it's trying to install for you. After you install this make sure that you restart your Garry's Mod and you will be able to see playx video!
http://get.adobe.com/flashplayer/otherversions/
http://get.adobe.com/flashplayer/otherversions/