By which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain. For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures; And that he was buried, and that he rose again the third day according to the scriptures..." I believe that this passage of scripture gives the simplest definition of what the gospel is.
I remember working with a fellow in Klamath Falls Oregon some years back. In one of our conversations he made a reference to "a come to Jesus meeting." I suppose that is what some would say is the result of preaching the gospel, someone coming to the Lord. The Lord Jesus did come into this world to draw all men to Himself. (See John 12:32) But the cross of Calvary was not "sloppy agape," it was a place of execution of criminals and it was no doubt an excruciating way to die. Yet this was the cause the Lord Jesus came into the world. In His humanity He said in Luke 12:50, "...I have a baptism to be baptized with; and how am I straitened till it be accomplished!" Yes Jesus came to die for us on the cross. It is no wonder that Apostle Paul, the writer of 1st Corinthians 15:1-4, said in 1st Corinthians 2:2, "For I determined not to know any thing among you, save Jesus Christ, and him crucified."
The gospel requires more than a mental accession e.g., accepting the Lord Jesus as your personal Saviour. That may be all some preachers and church organizations require but the Bible says in 2nd Timothy 2:19, "Nevertheless the foundation of God standeth sure, having this seal, The Lord knoweth them that are his. And, Let every one that nameth the name of Christ depart from iniquity." It is not enough to say "I know the Lord Jesus." The real question is does He know you? And in context of the foundation of God that Jesus knows you, have you and are you willing to "depart from iniquity?" The gospel that the Bible talks about requires repentance, e.g., a departure from iniquity. Apostle Paul told the men of Mars Hill in Acts 17:30 that while God may have winked at their ignorance in time past He now commands, "... all men every where to repent." The Lord Jesus, the central figure of the gospel, told the Jews in Luke 13:1-5, "...Except ye repent ye shall all likewise Perish."
In Acts 2:37 those Jews upon hearing the gospel asked, "Men and brethren, what shall we do?" The Apostle Peter's reply in Acts 2:38 was, "...Repent and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost." What is the Bible definition of the gospel found in 1st Corinthians 15:1-4? Is it not the death burial and resurrection of the Lord Jesus? For you and I it is death to sin in our lives through repentance (See Romans 6:11), burial in the saving name of the Lord Jesus (See Acts 4:12) and the promised resurrection power of God's gift, the baptism of the Holy Ghost. (See Romans 6:4)