Yeah its a difficult question, since GL has put a lot of time into the UI. It is extremely hard to enforce licensing on open source things, you need a team of lawyers if you really want to fight it in the end. The other company can just as easily rewrite the UI based on the GL one, so it works the same but with just different code, and have that running in 1 or 2 months, vs the time it took for GL, years to make the first one.
It is very common to use an open source system, with closed source code on top. Look at Apple for example, the system was based on BSD, and they had their UI on top. Nobody complains about Apple
If you need pure open source, there is Luci, it can do everything the GL UI can, just not as user friendly.
One thing GL could do is have a third party company do a code audit on the UI, and have certification that the code has been checked. This way it can remain closed, and have yearly checks of the code integrity for example
Also, GL does not use OpenWRT because its open source. They use it because its the leading router operating system. If there was a closed source licensed operating system as good or better than OpenWRT; im sure GL would be using that.