The Kingdom of God is the gospel. The kingdom is good news. The declaration of the Lordship of Jesus Christ is the power of God for salvation. Because Jesus rose from the dead, we are justified; we are saved. "Jesus is Lord" is true and because it is true there is salvation to be found.
And so this is a kingdom of grace. But that grace, that kingdom, comes with vengeance, for God will judge the secrets of men by Christ Jesus (Rom 2:16). All sin is going to be dealt with, either in Christ or outside of Christ. And all men, even Christians, are required to keep the law of God. That is why, when giving the Great Commission, Jesus said to teach them to observe all that He had commanded (Matt 28:20).
And even that is grace, a kingdom of grace. We have been saved so that we no longer are slaves to sin. We are able to keep God's law and even love doing so. We are able to hate our sin and make no excuse for it, instead, repenting of it. We are able to walk in the Spirit, putting off the effects and habits of the old man and putting on the character and practices of the New Man. In this sense, we become prisoners of Christ, under His command - a command of freedom and liberty to be what humanity was meant to be.
This is some of what Frame intends when he writes, "Kingdom...includes both grace and vengeance" (Frame, ST, p95).