I don’t know what has been tried so far, but here’s the outline of my procedure for fixing bricked Bluetooth firmware on the GTM in case you find it useful. I have sent a more detailed write-up to the wiki, should go live in a few days.
- get an Android device
- sideload a copy of the nRF toolbox from https://github.com/NordicSemiconductor/Android-nRF-Toolbox
- on your computer, download a copy of the goTenna APK (same version you were running at the time of the fupdate) and unzip it
- transfer only the files nrf52_*.zip to the Android device
- in the nRF toolbox, select the nrf52_phase1 file, then select the bricked GTM device (DFUTARG), click Upload and wait
- repeat for the remaining files in order
- if successful, the GTM will reboot in an operational state
Standard disclaimer applies, the instructions above may fubar your GTM, but since you’re already there, why not give it a desperate try 
and let us know how you go.