Port forwarding port overlap occured please help.

[Ams]

I just got gmod and when I go to make a listen server with create multiplayer game I follow the exact instructions my friend used to get his server up and for some reason no one can join my server. They say that it is not responding. Does it have anything to do with my css dedicated server? It shares the same ip but it isn't running when the gmod one is. Also when I try to use port forwarding and type in the ip and stuff I put the port 27015 in and click save it says port overlapp error has occured and wont let me do anything. I ahve a Linksys WRT350N. Please help me. Thanks