Defining "The Gospel" & "The Kingdom of God"

What is the gospel, and why is it "the power of God unto salvation"? [Romans 1:16, Mark 1:14-15, Exodus 19:5-6, Acts 1:6, 17:7, Colossians 1:13, Matthew 28:18, 13:31-33, John 8:31, Hebrews 4:12].