Can you play lan on gmod 13?
Yes, Yes you can!
Go to - Start new game
At the top right you will see (Single Player) Change the players!
Open up cmd (Command Prompt) type ipconfig
get your IPv4 Number it should be something like (192.168.1.4)
Then go back to your game and open console and type connect (IP) for example (conenct 192.168.1.4)
Hopefully you should be playing the server!