Fix video playback
The browser version that comes with the regular version of Garry's Mod was released in 2012 and is now out of date. To play videos that require modern web technology, you need to switch to the experimental branch of Garry's Mod.
- Close Garry's Mod if you have it open.
- Open your Steam Library.
- Search it for Garry's Mod.
- Press Right Click by game in the list and in the menu, select Properties.
- In the left pane, select BETAS.
- In the list Beta versions select x86-64 - Chromium + 64-bit binaries
Steam will download additional files and you can launch the game.
Read also: Enabling additional video codecs.