The video published by the developers shows that head movements are used to switch characters. The accelerometer built into the cap responds to them. To confirm the selection, click on the top of the cap. Thus, Gboard engineers replaced the entire keyboard with a single button.
The Bluetooth interface is used to connect the unusual headgear to a PC or smartphone. A USB-C port is provided for charging the gadget. The capacity of the battery pack built into the cap is 120 mAh.
The accessory will not be available for sale. If you want, you can assemble it yourself. The assembly instructions can be downloaded on GitHub.