Well I did say potencies and cool down times were hypothetical and would likely need rebalancing, it just gives a rough idea of how it would play.

As for how the gauge would fill in relation to hits taken and damage received.
I would imagine it could work as increasing by a base about for each hit taken, plus bonus for the amount if damage, as a percentage as you say.
Potentially something like 1 per hit, then .5 per 2% HP.
This would allow some increase even when taking no damage.

Not all the skills are locked behind the gauge though, the basic openers and weakponskils aren't, plus most of the cooldowns.