Hey guys,

Last night I decided to take my new GPD mini-comp to a small trip, with my Airfox K20 Bluetooth keyboard.
To my chagrin, the kernel did not find the Bluetooth firmware (I hadn't tested it before), with the following message:
Jul  7 04:46:40 doggy kernel: bluetooth hci0: Direct firmware load for brcm/BCM4356A2.hcd failed with error -2
So I used its own micro kbd, and went trolling for firmware, and found this most awesome link:

I clicked on the top link "brcm", and saw down the page a bunch of entries like:
 	BCM4356A2-0a5c-640a.hcd 	Version 	2 years ago
	BCM4356A2-0a5c-640e.hcd 	Version 	2 years ago
	BCM4356A2-0a5c-6419.hcd 	New version, released at 28.09.2016 	a year ago
	BCM4356A2-0a5c-6420.hcd 	New version, released at 28.09.2016 	a year ago
	BCM4356A2-0b05-181d.hcd 	Version 	2 years ago
	BCM4356A2-13d3-3473.hcd 	Version 	2 years ago
So, I clicked and downloaded BCM4356A2-0a5c-6420.hcd , just cuz it says "released a year ago", vs the other files' 2 year age.
So... I put this file in: /lib/firmware/brcm/ , and rebooted.

Voilla, works !