The author of 1 John writes, “Beloved, let us love one another, for love is from God.” (1 John 4: 7)
Paul encourages us to “speak the truth in love” so as “to grow up in every way into Him who is the head, into Christ.” (Ephesians 4: 15)
What does he mean to “speak the truth in love?”
“A soft answer turns away wrath, but a harsh word stirs up anger. The tongue of the wise commends knowledge… A gentle tongue is a tree of life… The lips of the wise spread knowledge.” (Proverbs 15: 1 – 2, 4, 7)
As we speak truth, knowledge, wisdom to our brothers and sisters, we are to keep a gentle tongue bathed in the love of God. We are to “restore [others] in a spirit of gentleness.” We are to “bear one another’s burdens, and so fulfill the law of Christ.” (Galatians 6: 1, 2)
Grace Partakers 
this is a good and challenging thought… To speak the truth, which is going to be corrective in itself, in love, you’re not making it worse, but being as gentle as possible with it so that the person doesn’t run the other way screaming… It is not always easy, it must be prompted by the Holy Ghost.
Yes, gentleness is essential in teaching, I think.