"In November 2002, Sword & Sorcery Studios (SSS) announced that it had licensed the Gamma World setting from WOTC in order to produce a sixth version of the game. SSS's version of the game, which reached the market in 2003, used the d20 Modern system, and mimicked D&D's "three core book" model with three hardcover manuals"
Source: Wikipedia, "Gamma_World", available under the CC-BY-SA License.