Connection failed after 4 retries

Hi
I have set-up my own server, by reading the following guide: https://www.youtube.com/watch?v=ShbdYM0xtT4
But no one apart from me can connect to the server, and my friends usually get the “Connection failed after 4 retires” error.
I have port forwarded properly, and it seems to only be GMOD that is not working, as I have set up a CSS server and my friends can connect to it.
Did I set up the server incorrectly?
Thanks

Your firewalls. You must set them up not to block GMod.