irst note that the GUI (and the integrated daemon (monerod)) will still function properly without any incoming connections. Now, to open port 18080 properly, you have to forward / open it in your router.
As for actually configuring your router to forward port 18080 to the machine on your local network; try to find some guides and/or your router's manual concerning port forwarding. In addition: Windows can block these requests so make sure the Windows firewall allows such connections (TCP and UDP).
Service: other
External host: (leave empty if you can)
External port: 18080
Internal host: (the IP address of your machine on your local network; it will start with 192.168.1...
Internal port: 18080
Protocol should probably be UDP (?) but if you can, choose 'Both' from the dropdown menu. print_cn will help you verify your incoming connections are working.