What is the Gospel of the grace of God?
The Gospel of the grace of God is the revealed truth that salvation is by faith plus nothing: believing that Christ died, shed His blood, was buried, and rose from the dead. God kept it secret until He revealed it, and now expects people to believe it, because faith comes by hearing the Word of God.
Les explains that the secret things belong to the Lord, and He reveals them in His own time. He rejects the idea that Adam and Eve or Abraham understood the Gospel, because God had not yet revealed it; it was kept secret in the mind of God. The Gospel of the grace of God was revealed later: all we have to do is believe that Christ died, shed His blood, was buried, and rose from the dead. When God said that is all that is necessary for salvation, He expects us to believe it. Now we are saved by faith plus nothing in what He has proclaimed. This comes back to Romans 10: faith comes by hearing the Word of God.