Aristotle used it in a technical sense as the virtue that strikes the mean with regard to anger: being too quick to anger is a vice, but so is being detached in a situation where anger is appropriate; justified and properly focused anger is named mildness or gentleness.
Gentleness is a strong hand with a soft touch. It is a tender, compassionate approach toward others' weaknesses and limitations. A gentle person still speaks truth, sometimes even painful truth, but in doing so guards their tone so the truth can be well received. "When my daughter was young, she used to love to squeeze my hand as hard as she could, trying to make it hurt. She could squeeze with all her might, but it never hurt. She didn't need to be gentle because she lacked the power to cause me any pain. Then, just for fun, I'd give her hand a tight little squeeze until she yelped. It's the strong hand, not the weak one, that must learn to be gentle."
A second important usage was common in medieval times, associated with higher social classes: hence the derivation of the terms gentleman, gentlewoman and gentry. The broadening of gentle behavior from a literal sense of the gentry to the metaphorical "like a gentleman" applicable to any person was a later development.