By comparing https://www.gl-inet.com/products/ with https://openwrt.org/toh/start?dataflt%5BBrand*%7E%5D=inet the OpenWRT support doesn't seem to be 100% but almost all of them are. I personally own an AR750S and a MT300.
Though the updating process can be more streamlined, I had to redo all my configuration when updating from openWRT 19 to latest 21 (could have been my own mistake during flashing, at least I have firmware updates).