Offizieller Kommentar
You're trying to send a line level signal to a mic-level input. The mic input on your GoPro is expecting a lower microphone signal which is then run through a mic amplifier, so of course the audio is being distorted.
You could use the booth output and run a lower level, but the EQ will still get all wonky because of the mic amplifier's curve. Use a proper output to record the audio and merge the two in post.