Along with GURPS Traveller: Star Mercs, GURPS Traveller: Ground Forces is the spiritual successor to Classic Traveller Book 4: Mercenary, providing rules and background information for the Marine and Army characters, including rules for designing grav vehicles, new military equipment, and a history of conflicts in the Traveller setting.
From Back Cover:
GURPS Traveller Ground Forces describes the Imperial Marines and Unified Armies, laying the foundation for a campaign thick with laser fire and full of action! It includes:
A history of ground combat and campaigns in the Imperium.
Detailed descriptions of Marine and Army units, from squads to battalions and up - as well as rules for easily determining the makeup of a world or subsector army.
New, expanded templates for Marine and Army characters.
New gear for Imperial soldiers.
New, modular rules for designing military and civilian grav vehicles in GURPS Traveller.