Bringing this back to the top in case anyone else is wondering if this is still the only way to do this in the latest generation of the GL.iNet routers, i.e. the Flint 2.
I can confirm that the only way to add additional WiFi networks (meaning new SSIDs in addition to the four that can be configured through the GL.iNet UI) is to edit the /etc/config/wireless file and add a new section to the bottom. Be sure to use a unique MAC address as well.
If you use LuCI to add a new wireless network, it only creates a partial configuration in the /etc/config/wireless file that is missing some crucial entries. But what worked for me was to start in LuCI to create the new Wireless network and Interface, then SSH into the router and complete the configuration in this config file.
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