PAUL THE APOSTLE PREACHED THE GOSPEL TO THE GENTILES.
He explains the Gospel in the Book of Romans and in 1 Corinthians:
Romans 1 verses 1 to 4:
“Paul, a servant of Christ Jesus, called to be an apostle and set apart for the gospel of God – the gospel he promised beforehand through his prophets in Holy Scriptures regarding his Son, who as to his human nature was a descendant of David, and who through the Spirit of holiness was declared with power to be the son of God, by his resurrection from the dead; Jesus Christ our Lord.”
Romans 1 verses 16 and 17:
“I am not ashamed of the gospel, because it is the power of God for the salvation of everyone who believes; first for the Jew, then for the Gentile. For in the gospel a righteousness from God is revealed, a righteousness that is by faith from first to last, just as it is written: “The righteous shall live by faith.”
1 Corinthians Chapter 15 verses 1 to 5:
“Now, brothers, I want to remind you of the gospel I preached to you, which you received and on which you have taken your stand. By this gospel you are saved, if you hold firmly to the word I preached to you. Otherwise you have believed in vain. For what I received I passed on to you as of first importance; that Christ died for our sins according to the scriptures, that he was buried, that he was raised on the third day according to the scriptures, and that he appeared to Peter and then to the Twelve.”
PETER PREACHED THE GOSPEL MESSAGE TO THE JEWS. 
See In Acts Chapter 2 verses 22 to 40 Peter preached the Gospel on the Day of Pentecost. Link to Peter preaching the Gospel. Comparing Peter and Paul's messages we can see that both gave the same gospel message. Both refer to the Old Testament prophetic message and the recent death, burial and resurrection of Jesus, all done according to God’s plan.
FF Bruce says: "Only one saving message is attested by the NT. The “gospel to the circumcision” preached by Peter and his colleagues did not differ in content from the “gospel to the uncircumcised” entrusted to Paul (Gal. 2:7), though the form of presentation might vary according to the audience. Paul’s testimony is, “Whether therefore it was I or they [Peter and his colleagues], so we preach, and so you believed” (1 Cor. 15:11).
The basic elements in the message were these: 1. the prophecies have been fulfilled and the new age inaugurated by the coming of Christ; 2. he was born into the family of David; 3. he died according to the Scriptures, to deliver his people from this evil age; 4. he was buried, and raised again the third day, according to the Scriptures; 5. he is exalted at God’s right hand as Son of God, Lord of living and dead; 6. he will come again, to judge the world and consummate his saving work."
For more Gospel references in Acts see 
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SO WHO IS THE GOSPEL ABOUT?
Note carefully from the above verses that Peter preached ALL ABOUT JESUS. Paul’s gospel was also ALL ABOUT JESUS. Paul said:"We preach Christ crucified....the power of God and the wisdom of God." (1 Corinthians 1:23-24). Consequently the gospel is not about man, nor how God feels about man. It is all about Jesus, from start to finish. Jesus is the gospel. John Calvin said:"the whole Gospel is contained in Christ".
TO SUM UP: According to the gospel, Jesus' coming was foretold by prophets, in accordance with God's perfect plan. Jesus willingly came from Heaven to save us, died for our sins in our place, was raised from death and was seen by many after his resurrection. He now sits at the right hand of God who has given him all power and authority, including the right to act as judge of all mankind. Since there is a judgment to come, the Gospel can also be described as "a rescue operation from the wrath of God".
WHAT ABOUT TODAY’S GOSPEL?
In recent years the gospel message is sometimes altered in a way that puts man into the gospel message given to people. For example:
“God loves you and has a wonderful plan for your life”.
(note than man gets two mentions here, while God only gets one and Jesus is not even mentioned at all!). For more on this, visit our "Man-Centred Gospel" Page
We need to beware of presenting a gospel that isn’t ALL ABOUT JESUS. Jesus IS the gospel! For more on this go to our "Dangers To Avoid" page.
WHERE DOES REPENTANCE FIT IN?
The way we RECEIVE the gospel is by repentance towards God and faith in the risen Lord Jesus. Repentance is towards God because the person you most offended with your sins is God himself. To repent means “to come to your senses about where you are, change direction and turn back to God.”
Chip Brogden says: "Repentance, in its truest sense, is an adjustment of the heart, an adjustment of the mind, the thoughts, and the attitudes of the soul; and this leads (or should lead), to an adjustment in terms of things we used to do that we no longer do, or things we have not been doing that we begin to do."
Rob Wilkerson writes: "Believe and repent are the two sides to the coin called "Conversion".
A person perceives and feels that what they have thought before is no longer the truth, but that what they have heard in the gospel is, in fact, true. This, in turn, is followed by a decision to stop thinking and doing what they did before, and start thinking and doing what Jesus tells them. When this happens, they 'convert' to Christianity.
So the gospel is a command to convert: "stop doing what you are doing now...stop thinking the way you are thinking now and start doing what Jesus tells you to do...start thinking the way God tells you to." The gospel brings a person to a specific point in time where a decision must be made on their part as to what their future course of action will be from that point onward. So then, conversion is decisive. It is also divisive. It requires them to divide themselves from their former way of life. This is why conversion is so difficult because it brings divisive consequences to such a decision."
WHAT IS THE CHIEF END OF THE GOSPEL?
John Piper in his online book God is The Gospel
says:"My point in this book is that all the saving events and all the saving
blessings of the gospel are means of getting obstacles out of the way
so that we might know and enjoy God most fully. Propitiation,
redemption, forgiveness, imputation, sanctification, liberation, healing,
heaven—none of these is good news except for one reason: they
bring us to God for our everlasting enjoyment of him. If we believe
all these things have happened to us, but do not embrace them for
the sake of getting to God, they have not happened to us. Christ did
not die to forgive sinners who go on treasuring anything above seeing
and savoring God. And people who would be happy in heaven if
Christ were not there, will not be there. The gospel is not a way to
get people to heaven; it is a way to get people to God. It’s a way of
overcoming every obstacle to everlasting joy in God. If we don’t want
God above all things, we have not been converted by the gospel."
APPPLYING THE GOSPEL
Read our page Applying It
