The letter to the Galatians is a letter written by the Apostle Paul to the churches of Asia Minor, in the letter he focuses on the problem of the "curse of the law" and the doctrine of Justification. At the end of his letter he reflects on love and its relationship with  the law of the return affirming: "Who sows in the spirit, will from the spirit reap eternal life", Galatians 6: 8.
Sowing in the spirit refers to life as in the Parable of the sower (Matthew 13) is living the spiritual gifts, to live according to the spirit, or as Jesus tells us, "Be perfect as the Father is perfect" Matthew 5:48. Spiritual gifts are not virtues, because these are inclinations of the soul, the spiritual gifts are perfections, excellencies.
Sow the spirit is to live according to God's plan, according to the promises made in the Beatitudes of the Sermon on the Mount. Sow in the spirit is perfect ourselves and become co-creators with God. And the Law of the Return acts, impiety returns impiety, justice returns justice, mercy returns mercy. When we sow in the spirit, when we are blessed and we perfect ourselves the law of return acts and returns to us Eternal Life.
The perfections return perfections, as well as a virtue returns us a virtue (justice returns justice) so the gifts returns other gifts. Eternal Life as a spiritual gift are blessings that God gives us in advance of the afterlife. Some of these blessings are: 1) Contemplative activity, 2) Gift of Happiness or happiness of the heart, 3) spiritual gift of health or longevity, 4) God's guide or third eye.
The end of the common people is happiness, "all beings desire happiness unanimously" Thomas Aquinas says. Eternal Life is more than happiness because one of its aspects is the happiness as a state of mind, in eternal life "being" and "happiness" are the same thing, when we have this kind of happiness, "exist" and be "happy" are the same thing, "because I exist I am happy", that is the true message of Christ in his Beatitudes, I am not happy because I have wealth, social status and possesions  but because I know God. True happiness is a product of  the contemplation of God. We are called not to the "opinated happiness" but to the true happiness, the happiness of the heart.