".....concerning His Son, Jesus Christ our Lord..." Romans 1:3
The gospel is a message that has the Lord Jesus Christ, as it's bright and bold centerpiece.
Christ is both preeminent in salvation and also in the gospel that declares how that salvation was accomplished. Christ is the gospel Himself, in that He is the good news, the answer, and the remedy for all that is lacking in a sinner's salvation. Jesus Christ is the one pointed to in the Trinity and when the gospel is preached.
Man is not worthy of mention when it comes to merit in the gospel. Man is the one being rescued and saved by Christ, the Savior. The only merit man has is negative: The wages of sin, which is death. Death is all man can bring to the table. When God's people have the Savior revealed to them they see their our unworthiness and all they can see is, "Worthy is the Lamb which was slain." They see Christ as He accomplished His service, finished the work, defeated death, hell, Satan, sin, and the grave, then to be resurrected and ascended to be exalted above any and all. Man continually seeks to suppress, tear down, and destroy the attention, preeminence, and honor that belongs to the Savior as they tried to steal center stage from Him.
Salvation is no doubt a Trinitarian salvation and has the involvement of the whole Godhead. God saw fit to have the second Person of the Godhead to have the bulk of the task that involved the most work which included humility, service, suffering even unto death because of sin. The Father, in His sovereignty, has seen fit to give preeminence to His Son. He is pleased with His Son as His Son glorifies Him always. The Son always had the will and honor of His Father in mind. In harmony with the Father, the Holy Spirit points to and reveals Christ. The Spirit does not point to Himself, but His task is to open the eyes of the spiritually blind to reveal the Savior, Jesus Christ the Lord. The Savior is shown to be the great gospel performer. The gospel concerns the Son of God.
Find a religion that takes away either the humanity or the deity of Christ and you have a false gospel. Find a religion that perverts His work as unfinished or failed (which is 99% of all religions) and you have a false gospel. Look at Judaism, which rejects Him as Messiah and you have a false gospel. Look at the Charismatic movement that are wrong on His Person and work, which in their case results in giving the Spirit preeminence. Little do all these people in these false religions know, they are of the antichrist spirit?
The gospel is not just everything in the word of God. It is a specific record of how Christ fulfilled all that it took to secure the salvation of God's people. That record is contained in the Scriptures and it the focal point of the whole Bible. This is the message that God has been pleased to use to reveal His Son to the elect. The gospel concerns the only qualified Person to do this work of wonder for His Father's glory and to deliver His people from their sin. He did it! Preach Christ and Him crucified and give Him the glory, praise, worship, and honor He deserves. God's people love this truth.