Multiplayer help

I wanted to play Gmod with a couple of friends, without using a LAN party because they werent near me, I tried joining my friends game but it said " Server Not responding" we tried multiple times, can anyone help me?

Did they open the correct port (default is 27015) on their router and or firewall.

Also tell them to use their WAN ip ( not your local ip from your router etc, your internet ip) if they are unable to find it themself use something like http://ipchicken.com

If they have a antivirus/ firewall software make sure they allow it through their firewall as well.