The Christian concept of humility is the understanding of complete and utter insignificance before the glory of God. Which coincides with the concept of God's infinite love (Why would a perfect being love something so broken? Because He is perfect, unlimited, beyond merciful and loving of each of us). We humble ourselves in worship, realizing that it is by no merit of our own that we deserve anything. But the graciousness of God. It's this understanding of our total reliance on God.
This doesn't necessarily mean we debase ourselves with other humans in trying to supplicate ourselves more and more. I think Aristotle kills this one with his concept of humility (he breaks it into two virtues but we won't get too technical). Basically the goal is to understand exactly where you stand with something, and assert yourself appropriately.
For instance, if you know a lot of math because you are a mathematician, and someone ask about it: you are going to be sort of an asshole if you throw yourself down as "I know nothing about math! I am such a fool" trying to be humble. But you also don't want to go "I am the best there is, you can't find someone better" since you probably aren't the best there is. And if you know very little math, to simply state that as well. Be willing to learn.
Rules of conduct often state that you don't announce your proficiency without it being relevant. It wouldn't help to just randomly drop in "Oh hi Dave. How was the weekend? Also I am way better at math than you." That is just bragging.
I still haven't worked out the appropriate time to interject if someone slanders Christianity. I tend to only do it if we are in direct conversation.
I think gentleness is a given. We are instructed to deliver the news graciously. Not slam someone over the head and try to make them cry. We aren't trying to win an argument (necessarily). We just want them to understand the truth and come to accept Jesus Christ. So defeating a bad argument is often part of that.