The common question what is the gospel probably should start with who is the gospel centered on? The gospel is centered on the person of God, through the person of Jesus Christ. You see Jesus Christ has been commissioned by God the Father to bring to himself a kingdom, and by bringing to himself a kingdom, he is making kingdom citizens. All the way back in the Garden of Eden, mankind fell into sin and Christ has now come as predicted in the Old Testament and shown in the New Testament, he's come as the Savior to save mankind from sin, and for all those who would believe on the Lord Jesus Christ, repenting of sin and coming by faith to him, he makes them into kingdom citizens. As the kingdom citizenry on earth is building and building, there will be a day when Christ returns, sets up his kingdom on earth, and the gospel will now find its consummation in the kingdom of Christ.