Even it's an older topic...but as I had this task today, here the way it worked for me on an AR300M16-EXT:
uploaded and installed 22.0.3 "Kernel"-Image over the GUI of the original GL.Inet software (retain config was NOT ticked)
after reboot was done, OpenWrt was reachable over 192.168.1.1. In the top there was a message "System running in recovery (initramfs) mode.". At this point I used the 22.0.3-SYSUPGRADE-image to install the firmware using the normal System->Flash firmware way.
after another reboot everything seems to be up and running.
If you would like to remove the GL-iNet software and only use openWRT. Your best friend is going to be the usage of Uboot. Unfortunately, it's a very advanced process for some models. Easier on others. Please read this guide carefully.I suggest at least twice.
You can easily brick your device. This is not for the faint of heart or wallet.
I just successfully installed the 22.03.3 nand-squashfs-sysupgrade.bin directly onto my GL.iNet GL-AR300M-Ext using its web interface after having updated to the latest firmware 3.215 from the GL.iNet downloads page. This worked right away without any problems or additional steps. Of course I did not attempt to keep the settings.