Get message from Gadgetbridge

I'm guessing switching between apps like that isn't possible with gadgetbridge yet?

I think it might be - at least for some players. Gadgetbridge can now send 'intents': http://www.espruino.com/Gadgetbridge#int­ents

And many Android apps provide ways for you to send an intent to get them to start up and do what you want. Worst case you could just send an intent to Tasker, and get Tasker to do what you want