Gentleness is the opposite of the bad behaviors in 5:20 and requires strength (see Proverbs 15:1, 4; Matthew 11:28–29; Ephesians 4:2).
Self-control: The Holy Spirit does not allow immoral behavior but helps people avoid sin (compare Galatians 5:13; Romans 6:14–18; 1 Thessalonians 4:3–7; 1 Peter 2:16; see also Acts 24:25; Titus 1:8).
Against such things there is no law: Paul explains what is already obvious, that those led by the Holy Spirit do not need laws to guide them.