Connecting to a local gateway is almost exactly the same as a remote gateway. If it's not automatically detected, you'll have to select manual and try entering the IP address and port of the remote gateway. For instance if the gateway's IP address was 192.168.1.20 and you're using the default ports, you would enter http://192.168.1.20.8088 for the gateway address and it should add it to the designer launcher.
If you're using Windows, you may need to open the firewall for port 8088 to connect from another computer.