Connection failed after 4 retries

I have set-up my own server, by reading the following guide:
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?

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