He begins his list here because humility is essential to unity. Pride almost always results in discord. Think about it for a moment. The people we tend to like are the people who show us the respect we deserve, or at least that we think we deserve. The ones we don't like are the ones we sense don't like us, or who won't put up with our particular quirks.
Gentleness is a tricky word.
It's the word that used to be translated meekness. It's the quality of moderation, the golden mean. It had the idea of strength under control. So it's the characteristic of a strong personality who doesn't let their strength control them, nor use it to control others. Rather it's a strength that's there to serve others.