Level Gap XP Bonus, where is it?

[A-Shock]

It's included in your XP from mob kills, yes. It doesn't shows too overtly, but you will notice that every mob you kill has a "(+50%)" after it's exp, that 50% bonus is from the armory bonus

I don't think this is entirely accurate. The higher the difference between your highest level character and the one you're leveling depends on the bonus. So maybe it's 50% for a certain level difference. But it could be more or less. I don't know the min/max of it though.