Rage is the command ability for Gau, which allows him to copy a monster's stats, elemental and status resistances, innate abilities, and a couple abilities, but places Gau under a Berserk-like state until he dies.
Use Berserker skills.
Instinct is the command skillset for the Berserker class.
Savagery is the command skillset for the Berserker class.
Rage (often called fury or frenzy) is a feeling of intense, violent, or growing anger. It is associated with the fight-or-flight response and often activated in response to an external cue, such as an event that impacts negatively on the person.