Can't play online with a friend.

I can connect to other servers (and so can he) but when either of us creates multiplayer we can’t join eachother’s server. It says that the server is not responding. What is wrong?
We both have authentic gmod copies.

This is due to steam retching your local IP (192.168.X.X) When your friend needs to connect with a WAN IP (X.X.X.X)

Solution is to find your real ip:

You need to open ports on your router to play together online.
The exact port is 27015.
For more info, go to the server hosting part of the forum.

Thanks, I’ll try it out and see what happens.