Multiplayer Issue

So my friends and I just want to start a standard multiplayer server by making one with the “Start Multiplayer” option. Now when one of my friends try to join my game it just says “Server is not responding” now I am sure this is because I need to open ports but is there any way around that? The reason I am asking is because I have opened ports with other games with no success. If there is a program that gets around it and you are sure that it is compatible with Gmod please let me know. Or a pic of what the opened ports should look like would also help tons. Thanks in advance.

You do need to open port 27015. If there is an option for port triggering instead of port forwarding use that. You also need to use a static IP. Just use google for a tutorial.

Edit:
It also may be a problem with your ISP because some ISPs dont allow ports to be forwarded due to security reasons.

I have a dlink router so when I am on it there is a drop down list and Half-Life is there would that work as it fills in all of the fields? The TCP that it fills out is 6003, 7002 and UDP is 27005,27010,27011,27015

Give him your external IP and tell him to open the console and then type “connect <ip>”. <IP> being your IP.
If you’re not sure what your IP is, go to http://www.whatsmyip.org/ .