When you launch GMod after applying the fix, does macOS's Console.app say anything? There should also be a file called chromium.log in GMod's install that should have more info
Everything the console says after applying the fix:
No Files Need Fixing!
CEFCodecFix applied successfully! Took 0.9721 second(s).
Do you want to Launch Garry's Mod now? (yes/no)
>>> y
Launching Garry's Mod:
open steam://rungameid/4000
Press Enter to continue...
Saving session...
...copying shared history...
...saving history...truncating history files...
...completed.
Deleting expired sessions...none found.
[Process completed]