Hosting a Gmod "Listen" server, please help D:

Hello, I’ve been trying for what seems like… 35 years, yes that seems right, to make a Gmod server.
(To clarify I don’t want to make a dedicated server, I believe people call it a listen server, where you just click “create multiplayer” in game)

So anyways, I must say I’ve tried nearly everything, and I must be doing something wrong.

Here is my situation:
I am on a computer that is in a network of 3 computers including my own.
The network has a main server.
The router is a Netgear, I’m not sure what model exactly.

I’m not sure if any of that information helps.

If you can help me to get this work I will be so grateful, like, unbelievably grateful.
If you can take time to work it out with me over steam please add posdrums

[Please, oh please, do not just ask me questions of what I have done. Nothing I have done has worked, obviously, and I find it’s more irritating to ask a bunch of questions than it is helpful.]

I will try to find out the model of the router if it is necessary.

Forward your ports, also sv_lan 0.

Care to elaborate?

Configure your router to forward port 27015 UDP to your computer. There are many tutorials on how to forward ports, search around on Facepunch/Google.

Also, make sure to enter the command “sv_lan 0” into your console (without quotes).

Do I make the IP adress on the portforward mine, or the servers, or what?