Gamma World Game Master's Guide (2004)

Rules mechanics based on the D20 Modern RPG

From the back of the book:

It's a big world, with many secrets waiting to be revealed, from the changes wrought by war and disaster to cryptic alliances pursuing sinister goals. This book reveals these secrets, and gives you solid, practical advice for your Gamma World campaign. Learn how to modify rules for the right balance of adventure and danger, how to plan and run a campaign from exciting start to rousing finish, even how to incorporate additional mutations and new technology. You've got good ideas - put them to work!

Note: Werner Hager is credited as "Werner Hagen." Ian Harac is credited as "Lizard." Ellen Kiley is credited as "Ellen P. Kiley."