Self-control literally means the ability to direct one’s will. This is also known as willpower – keeping our emotions, impulses, or desires in check. The self in us is very strong, and that's what makes self-control difficult. But self-control is far more than the exertion of sheer discipline to keep from doing certain things...like eating donuts. Self-control is really about submitting your will to Jesus, then allowing His Spirit to direct your life.
So why does Gentleness make it into the list of fruit that the Holy Spirit develops in us? What was the Apostle Paul thinking when he included it? He was thinking of Jesus, and how this character trait was modeled first in his life and then in the lives of his disciples, paving the way for the church to embrace it as well.