If you are looking for "pieces" in the sense of campaign content or modules, here are the core classics:
: The Legion of Gold Expansion Pack includes four additional sheets of mutant monster tokens to expand your table presence. Adventure Modules & Supplements Gamma World Adventures
: If you are playing the 7th Edition (2010) , the Gamma World Starter Box comes with sheets of mutant monster punch-out tokens and reusable battle maps. If you are looking for "pieces" in the