I have been a Gamma World fan since the 1980s, and have seen every edition come and go. Within my heart burns the desire to run a Gamma World campaign, but like many others have never settled on a system (actually I have - Savage Worlds - but that is another review).
This book in the somewhat marred 6th edition D20 lineup, provides an excellence of depth and storycrafting for various secret societies (Cryptic Alliances) and oddball communities that may populate the post-Apoc wastelands. I highly recommend it as background source material - you won't be disappointed with the great ideas contained therein (much as the Game Masters book was also chock full of good ideas. Simply ignore/convert the D20 rules into your fav system, and enjoy!).