From publisher blurb:
D&D Gamma World provides you with dozens of monsters to use in your games, both in the original boxed set and in Famine in Far-Go. However, it lacks rules on how to build your own custom monsters rules that are – located throughout several books in Gamma World's sibling game, Dungeons and Dragons.
This document is designed to help you, a D&D Gamma World Game Master, create your own monsters from scratch and use them in your games. Options are also provided for random monster creation when appropriate, and some of the formulas used in monster creation are shown in gray boxes.