From the official GKOS website
GKOS - a new category of full-featured physical keyboards:
Not QWERTY, not exactly Wearable - but for Small Wireless Devices!
The basic idea of GKOS is that you've got six buttons on the back of a device (either a dedicated input device, or on the back of a handheld computing device, phone, etc), one for the index through ring fingers of each hand (or two for each finger if you have a device made for one-handed use). You press the keys in various chords to type, switch between modes, and operate a pointer (when in pointer mode).
This project is an implementation of GKOS for Linux (and soon to be for AVR microcontrollers), made mostly so I can try out GKOS for myself.
The Linux version (currently the only version), takes over a standard USB HID keyboard (most likely a USB number pad) via libhid, and uses 6 of its keys to function as a GKOS keyboard, sending keypresses via libsuinput. It will be configurable (right now it's "configurable" by changing a couple of lines in main-linux.c) at some point in the near future.
The AVR version will work together with V-USB and/or LUFA to implement an actual HID keyboard/mouse, and therefore won't need drivers for any OS.
The current implementation has the basic key scanner implemented (meaning you have to let go of all keys before a keypress is sent), with nearly all of the characters in the spec implemented (a few international ones aren't done because they aren't things that can be typed by just sending a keycode or SHIFT+keycode, I'll add in Compose support at some point, probably to make those work).