The latest patches by Marek implement ARB_texture_gather and gather-related ARB_gpu_shader5 functions from the GLSL 4.00 specification. With the patches, all the related Piglit regression tests to these functions now pass.
As explained by the OpenGL.org registry, "This extension provides 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."
Outside of the Radeon driver, there's been Intel texture gather patches going back one year and the core Mesa support was committed last October and in February was when texture gather support came to Gallium3D.
The patches implementing the ARB_texture_gather support for the RadeonSI driver to support the extension on AMD Radeon HD 7000 series hardware and newer is currently found on the Mesa-dev mailing list but should work its way into Git for the Mesa 10.3 release. Besides the Mesa work, the texture gather support does require some patches to the AMD R600 GPU back-end within the LLVM code-base too. Hopefully those AMD LLVM back-end improvements will land for the LLVM 3.5 release this summer.