The word gospel simply means “good news.” The gospel of the kingdom is the same gospel that Paul preached. Christ put it this way: For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life. It is the news of repentance, redemption, and restoration.
John 18:36
Matthew 4:17
Luke 10:9
Mark 1:14–15
Luke 17:20–21
Romans 14:17
As far as there being a secret second gospel Paul warned:
Galatians 1
6 I am amazed that you are so quickly deserting Him who called you by the grace of Christ, for a different gospel; 7 which is really not another; only there are some who are disturbing you and want to distort the gospel of Christ. 8 But even if we, or an angel from heaven, should preach to you a gospel contrary to what we have preached to you, he is to be accursed! 9 As we have said before, so I say again now, if any man is preaching to you a gospel contrary to what you received, he is to be accursed!