GURPS War Against the Chtorr is a GURPS Third Edition setting based on David Gerrold's War Against the Chtorr novel series, pitting humanity against alien invaders bent on transforming Earth's environment into one matching their home planet.
From Back Cover:
What do the worms eat? Bushes. Trees. Dogs. Cows. Cars. And people. Especially people. The worms eat everything slower than they are. And the worms are very fast.
David Gerrold's classic novels of The War Against The Chtorr form the background for this gripping game-world of heroism and disaster. Here you'll find:
A complete description of the post-invasion Earth.
Character and campaign ideas. Will you fight back against the invaders, or prey on your fellow humans as the world changes? or will you become a renegade, helping the worms as your own body becomes alien?
Dozens of Chtorran creatures, from stalking terrors that swallow you whole to swarming horrors that gnaw you to the bone.
Terran countermeasures: super-robots, Lethetic Intelligence machines, and the Telepathy Corps.
Chtorran jokes. ("What does a Chtorran call a pacifist? Lunch.")