why does gmod have to be so forgetY
gmod has nothing to do with it
Networking all works the same way, it's all handled by your router, the game is irrelevant.
these sites never work for me. i have ports that are clearly, provably opened and they all come up as closed
Did you forward TCP, or just UDP?
Games are going to require UDP, but these sites are only going to test TCP, because the differences between the protocols requires that you have an application running and listening for requests