What makes AOSP a better and more meaningful target for a truly open source system is being also open source and having a much better userland implementation that is supported by specific kernel patches that isolate client programs from direct hardware access and from each other. Moreover it has better libraries for creating mobile apps. It has a better and established ecosystem full of experts, some of which already implement FOSS apps.
No GNU system has anything close and getting close is a huge engineering effort that you don't need to do.
What AOSP does is actually workaround or extend Linux kernel with a system layer that makes the runtime for apps look more like a microkernel. It is a much secure design for carrying a general purpose computer with GPS, Camera and Bluetooth on.
So why not just improve Apache-licensed AOSP? Port the closed drivers to open source ones and everyone can utilize all of AOSP's benefits, without being controlled by Google or phone manufacturers too (as long as you can modify the software). FOSS friendly phone vendors can always help with porting efforts and remove blockers.