The last version of the v5 series of GNU Chess was 5.08, released in 2011. By that time, Fabien Letouzey had developed Fruit, which proved to be a stronger chess engine and shaked the computer chess world. At some point in time, Fabien kindly assigned copyright to the FSF, which allowed us to make Fruit 2.1 (the latest free version) the base for GNU Chess v6.
Therefore, it can be fairly said that Fabien Letouzey is remarkably the main contributor to GNU Chess v6.