The kernel and several of the userspace libraries are common open-source projects: Linux, libusb, bluez, and so forth. Some other parts of the HAL are shared with AOSP: GPS, camera, and others. You could call Gonk a very simple Linux distribution.
Gonk is a porting target of Gecko (just as Gecko has been ported to OS X, Windows, and Android). Since Firefox OS has full control over Gonk, we can expose interfaces to Gecko that aren't accessible on other operating systems, for example the full telephony stack and display frame buffer.