The Tactics Gauge is a meter displayed on the left of the screen when playing a scene in Gungnir. Each time a unit moves, the gauge is filled with Tactics Points according to the unit's Tac. value; if the unit's Tac. is 3, they can increase the Tactics Points by three if they move at least three spaces. Certain moves and overall damage output are optimized by a high amount of points in the gauge, so it is recommended to keep it high as much as possible.
Using actions such as Boosts, Beats, and Scramble (see here for details) consumes Tactics Points. Frequent use of these mechanics can and will deplete the gauge quickly. Therefor, the player must be wise about when such actions are used, and be sure to use custom beat/boost strategies to avoid unnecessary point usage.
There are a handful of consumable and equippable items that help build the Tactics Gauge faster. The Art of War item for example will increase the gauge's meter by ten (10) pts when used. Equip items such as the Intelli Ribbon increase the Tac. value of the wearer. The player should take advantage of these items as soon as possible, as they can make battles a bit easier.