Well, there goes the "no need to buy the WHIM" idea 
I can only speak to my experience. When I installed the GTS on my 15 SG, out of the box, before having it flashed, when I plugged my iPhone into the usb, Carplay would come up. With a plugged in headset in I could use the Boom or Siri speech commands.
What I didn't try, Is what would happen if I had my Sena paired to the iPhone while plugged into the Boom usb.
I'm very impressed with the functionality of the Boom GTS. I have since installed the WHIM. As of to date the stuff just seem to work.
*HD Sena paired via BT iPhone
*HD Sena paired via BT to GTS Boom
*Iphone paired via BT to GTS Boom
*Iphone plugged in Booms usb (makes Carplay available)
Seems like a lot of pairing going on but like I said, when I hop on the bike whether I have my headset or not, Iphone plugged in or not, so far it does what its suppose to due.