ARB_texture_gather is an OpenGL extension required by version 4.0 and described by the OpenGL.org registry as "a new set of texture functions (textureGather) to the shading language that determine 2x2 footprint that are used for linear filtering in a texture lookup, and return a vector consisting of the first component from each of the four texels in the footprint."
Radeon and Nouveau Gallium3D drivers will finally be able to tap this texture-gather support originally developed by Intel for their own driver. David Airlie of Red Hat hooked in the support for Gallium3D via this Gallium commit and this Mesa state tracker commit.
The new code also works towards enabling (but not yet completed) of the GL_ARB_gpu_shader5 extension. These changes will be part of Mesa 10.2 and not the Mesa 10.1 release due later this week.