From Alternity Gamma World Campaign Setting introduction:
The ALTERNITY rules system is a roleplaying game system of modern action to far-future adventure. It is a ruleset that allows players to enjoy heroic adventures in the worlds of science fiction, and in the case of GAMMA WORLD, science fantasy. The STAR*DRIVE and DARK-MATTER campaign settings use the ALTERNITY game rules as well, and heroes from one setting can be readily transplanted to another.
ALTERNITY is also available as its own set of hardbound rulebooks - the Player's Handbook and the Gamemaster Guide.