How to make Gmod server

what you want to do is make sure you have a reliable connection and you PC or dedicated server meets the requirements for it
what you want to do first is grab yourself the program steamcmd
second type in login anonymous
third type in "force_install_dir c:\my_garrysmod_server\"
fourth type in "app_update 4020"
and your server will download and install to you c drive
and for portforwarding i would have to take a look at what router you are running