The GK20A graphics is the Kepler-based GPU found within NVIDIA's Tegra K1 SoC. The Tegra K1 is quite a beast with four Cortex-A15 CPU cores, a fifth companion core, and Kepler graphics that use NVIDIA's mainline Linux graphics driver with support for full desktop OpenGL 4.x and CUDA. The Tegra K1 is what powers the interesting NVIDIA Jetson TK1 and will be powering many other consumer devices in the months ahead.
Back in February is when NVIDIA provided open-source GK20A / Tegra K1 graphics support on the kernel side by adding the DRM/KMS support for this Kepler-based GPU to the Nouveau DRM driver. NVIDIA just didn't do an initial code drop but the work went through the review process and got things working quite nicely on an open-source Nouveau-based driver stack to complement their proprietary Linux graphics driver that's part of the Linux 4 Tegra image. The GK20A Nouveau DRM code will likely land in Linux 3.16.
NVIDIA fortunately didn't stop with the DRM/KMS support for the Tegra K1 graphics but now they've posted a minor set of Mesa modifications that allow the GK20A to work with the NVC0 Gallium3D driver atop the supported DRM driver. Alexandre Courbot shared that with these two patches it's "possible to run Mesa programs on GK20A (Tegra K1). The support was rather trivial on the Mesa/Gallium3D side since the GK20A is similar to the already-supported GK104 graphics processor while using the GK110 ISA.
The GK104 GPU is what's in the GeForce GTX 660 Ti through GeForce GTX 680 series graphics cards; GK110 meanwhile is what's in the GeForce GTX 780 and TITAN graphics card series. The small Nouveau Mesa patch-set for the GK20A / Tegra K1 support is currently queued up on the Mesa-dev list and has already been reviewed by frequent Nouveau contributor Ilia Mirkin. Hopefully this support will land for Mesa 10.3 as it's now too late for the imminent Mesa 10.2 release.
Update: The support has now landed.