Don't know what GCE is or which books you need?
The GCE page on tUGS has answers you seek.

GCE Core Rules

Rules for battles in the GCE system.

By Da GCE Kommittee

GCE Campaign Book

The common campaign framework for GCE.

By Da GCE Kommittee

GCE Scenario Book

By Da GCE Kommittee

Gorker & Morker Faction Pack

A faction pack for fielding Gorkers and Morkers in GCE campaigns.

By Da GCE Kommittee

Digga Faction Pack

A faction pack for fielding Diggas in GCE campaigns.

By Da GCE Kommittee

Rebel Grot Faction Pack

A faction pack for fielding Rebel Grots in GCE campaigns.

By Da GCE Kommittee

Mutie Faction Pack

A faction pack for fielding Muties in GCE campaigns.

By Da GCE Kommittee