A Gravitic Lance is a now-outdated, early gravity-based weapon. It is a short-ranged (1000 KM) device which sets up an interference pattern in the defensive gravity shield protecting a ship equipped with a gravity drive, causing the enemy vessel’s gravity drive to overload and shut down, rendering the target vessel vulnerable to attack with conventional weapons. It can also be used to make a jump point collapse, causing the enemy's grav drive to explode and destroying the target in the process.
Unfortunately, it was found too difficult to make the grav lance system small enough to shoehorn into any warship smaller than a battlecruiser without an unacceptable penalty in other armaments having to be removed to make room... and any ship powerful enough to get close enough to a large gravity-drive equipped enemy warship to fire the darn thing probably didn't need some fancy Grav Lance to take the vessel down. Consequently, the Gravitic Lance was only ever installed on Revenge-class battlecruisers.