Mesa patches for ARB_texture_gather have been floating around on Mesa development channels for many months, but only today was the full support committed to Mesa master. ARB_texture_gather is a requirement of OpenGL 4.0 compliance and is 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."
The ARB_texture_gather support affects various areas of Mesa and spanned several commits including changes for GLSL, core Mesa, and the individual drivers. ARB_texture_gather in Mesa today only is supported by the Intel i965 DRI driver and for there only found with Ivy Bridge and Haswell processors. Support for the other drivers (namely Radeon and Nouveau) will come in due time.