I'm not sure if this is a bug or not but I notice the the gl.inet firmware install package does not include LuCI. Is this intentional?
normal, if you installed an openwrt snapshot.
Are we talking about OEM firmware, or firmware from downloads.openwrt.org?
Ah, that would explain it. It also probably explains why the Wireguard configuration in LuCI behaved a bit oddly. After setting up the peer settings and saving, the peer data was not saved. You could see the peer but the settings were the empty defaults. After a couple of tries it worked.
I used the firmware linked from the XE300 hardware page https://openwrt.org/toh/gl.inet/gl-xe300
Looking at the downloads it appears XE300 is not currently supported as a release version.