The bounty is in the amount of $2,000 USD, which isn't much at all considering the workload, but it's available to any individual willing to do the work. To bring Gallium3D to Haiku OS it would require first porting over Mesa to Haiku's architecture, port the Nouveau Gallium3D driver, and write the necessary winsys for Haiku. For the Gallium3D support it will also mean porting over the GEM/TTM memory manager to Haiku as well.
While Gallium3D is primarily used on Linux, it is compatible with Windows as well, it's being ported to BSD, and Gallium3D has even been ported to AROS.
Those interested in this Gallium3D-Haiku bounty project, visit HaikuWare.com -- there's certainly more than a few Phoronix readers capable of doing the work.