Why does Paul call his message the Gospel of the Grace of God?
The reason is that where sin abounded, grace did much more abound. God loves even the ungodly and can reach below the vilest sinner, lift him up, and make him a saint. Grace therefore triumphs over sin, so Paul refers to his message as the Gospel of the Grace of God.
Les explains that where sin abounded, grace did much more abound. Even someone who seems ungodly, hateful toward God, living like a reprobate, and avoided by others is still loved by God. God is able to reach below the vilest sinner, lift him up, and make him a saint. That superabounding grace is why Paul refers to his Gospel as the Gospel of the Grace of God. Les then moves to Romans 10:9-13, stressing that salvation involves verbal confession, not merely mental assent; a person should tell God out loud that he is a sinner.
Moreover the law entered, that the offence might abound. But where sin abounded, grace did much more abound: