You can follow progress in the next-update channel of the GMod Discord. The GMod devs are working on a fix by updating the game to use Chromium/CEF. This is normal, and a result of deleting 'htmlchromium.dylib'. As for why this mod broke, YouTube has stopped supporting old video encodings which GMod relied on. Launch the game, and you will notice that there is no main menu.

On the drop down box select the x86-64 - Chromium + 64-bit binaries option. Delete the file, and exit out of the directory. This is a short guide on how to switch your default Garry's Mod to version x86-64 - Chromium + 64-bit binaries. For some reason, this makes GMod crash if it is inside the game. I would really like to use ReShade with the 64. ReShade should be installed to the Gmod.exe file there, and it works just fine Keeping the original question and post up if anyone else googles the same question. EDIT: The solution is simple, I didn't notice the 64 bit sub folder in the bin folder.