No seriously, the radio is not connected to the computer system, the comptuer system is extremely conservative by many standard and is not connected to the in flight wifi. You cannot have an air-gap attack without a microphone or similar device.
The gao report is a complete nonsense and was laughed out by all technical people involved in the computer system of airplane or in flight entertainment.
"So, Mr cyber Expert and Pilot, other than saying "nuh uh", do you have anything to suggest there is no chance of this?"
Aside complete network separation and absence of microphone ? Really guys sometimes there is absolutely NOTHING about threat reports.
Hackers of the world, unite!