Beast Gauge
Official Lodestone Job Guide (Always up-to-date)
Beast Gauge is Warrior's Job Gauge.
Beast Gauge
Upon learning the action 
Defiance (tank stance) at level 10, a symbol will appear on the screen indicating when it is in effect. Defiance increases enmity gained from using other actions.
Upon learning the trait 
The Beast Within, the symbol which indicates the use of Defiance will change to also display the Beast Gauge.
The Beast Gauge displays a warrior's current amount of wrath, which is charged by executing certain weaponskills and by the ability 
Infuriate.
Wrath is used to execute actions such as 
Inner Beast, 
Steel Cyclone, 
Fell Cleave, 
Decimate, 
Chaotic Cyclone and 
Inner Chaos.
Simple Mode
Defiance (Active)
The gauge cannot be displayed in simple mode until learning the trait The Beast Within at level 35.