The true gospel is the good news that God saves sinners. Man is by nature sinful and separated from God with no hope of remedying that situation. But God, by His power, provided the means of man’s redemption in the death, burial and resurrection of the Savior, Jesus Christ.
As a result of the fall of man in the Garden of Eden (Genesis 3:6), every part of man”his mind, will, emotions and flesh”have been corrupted by sin. Because of man’s sinful nature, he does not and cannot seek God. He has no desire to come to God and, in fact, his mind is hostile toward God.
Romans 8:7-8 NKJV
Because the carnal mind is enmity against God; for it is not subject to the law of God, nor indeed can be.  So then, those who are in the flesh cannot please God.
God has declared that a lost man’s sin dooms him to an eternity in hell, separated from God. It is in hell that a lost man pays the penalty of sin against a Holy and Righteous God. This would be bad news indeed, IF there were no remedy for the penalty of sin.
But in the gospel, God, in His mercy, has provided that remedy, a substitute for us”Jesus Christ”, who came to pay the penalty for our sin by His sacrifice on the Cross.
This is the essence of the gospel which Paul preached to the Corinthians.
1 Corinthians 15:1-5 NKJV
Moreover, brethren, I declare to you the gospel which I preached to you, which also you received and in which you stand,  by which also you are saved, if you hold fast that word which I preached to you-unless you believed in vain.  For I delivered to you first of all that which I also received: 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,  and that He was seen by Cephas, then by the twelve.
He explains the three elements of the gospel. The death, burial and resurrection of Jesus Christ on our behalf. Our old nature died with Christ on the Cross and was buried with Him. Then we were resurrected with Him to a new life.
Romans 6:4-11 NKJV
Therefore we were buried with Him through baptism into death, that just as Christ was raised from the dead by the glory of the Father, even so we also should walk in newness of life.  For if we have been united together in the likeness of His death, certainly we also shall be in the likeness of His resurrection,  knowing this, that our old man was crucified with Him, that the body of sin might be done away with, that we should no longer be slaves of sin.  For he who has died has been freed from sin.  Now if we died with Christ, we believe that we shall also live with Him,  knowing that Christ, having been raised from the dead, dies no more. Death no longer has dominion over Him.  For the death that He died, He died to sin once for all; but the life that He lives, He lives to God.  Likewise you also, reckon yourselves to be dead indeed to sin, but alive to God in Christ Jesus our Lord.
Paul tells us to “hold firmly” to this true gospel, the only one which saves. Believing in any other gospel is to believe in vain.
In Romans 1:16-17, Paul also declares that the true gospel is the power of God for the salvation of everyone who believes, by which he means that salvation is not achieved by man’s efforts, but by the grace of God through the gift of faith.
Ephesians 2:8-9 NKJV
For by grace you have been saved through faith, and that not of yourselves; it is the gift of God,  not of works, lest anyone should boast.
Because of the gospel, through the power of God, those who believe in Christ (Romans 10:9) are not just saved from hell. We are, in fact, given a completely new nature (2 Corinthians 5:17) with a changed heart and a new desire, will, and attitude that are manifested in good works. This is the fruit the Holy Spirit produces in us by His Power.
Works are never the means of salvation, but they are the proof of it (Ephesians 2:10).
Those who are saved by the power of God will always show the evidence of salvation by a changed life.