As of now GZDoom is licensed under the GPL.
Over the last 3 days I rewrote the last piece of code, the middle layer of the OPL player to be free of MusLib. This was achieved by picking equivalent code and data structures from Chocolate Doom.
In order to complete the license transition the FMod sound backend also had to be removed. As of now OpenAL is the only remaining option for sound playback.
I have been using the OpenAL backend exclusively for the last few months and so far it hasn't shown any problems, so I am relatively confident that this won't pose a stabilty issue.
Note that no license headers in the source have been changed yet, this will be done over the next few days.