Just to make sure we explain correctly, what exactly are you referring to by "Mystery Gauge"? Do you mean the bar your characters have during battle that lets them use a super attack when it's full?
Mystery Gauge = Orange bar under your character panel lets you use Mystery Attack aka Ougi OVERDRIVE = Enrage meter on bosses aka OD in our group
Mystery Gauge generation works by normal attacking affected by Double/Triple attack gains also there are skills that increase gains I.E Asura/some character skills/support skills Mystery Attack aka Ougi = High damage cut in attacks that vary according to weapons. Some weapons will have debuffs and some will have buff or nothing at all. Damage is also listed as Very Large/Large/Medium
OVERDRIVE = a building meter everytime you attack the boss in overdrive state the boss will have different abilities/more damage with those abilities. So if you get lucky and it uses the normal super(full turn attack when all dots are filled) it'll do more damage in OD mode.
Not sure if any of the mods still support this community, but they should be aware that the forums are infested with bots and that their forum needs to be upgraded.