The greek word translated into English as "gospel" in the New Testament is rightly defined as "good news". And there is no greater news than the gospel of Jesus Christ.
So what is the Gospel?
If you ask 100 people this question you will likely get 100 different responses.
Read the article "Reflections On The Gospel" by Rod Graciano below to help you clarify what the Gospel is, then read Paul's first recorded gospel message in Acts 13:16-41. Comment below where you see Paul preaching the 4 core truths of the Gospel in this passage of scripture.
Acts 13:16–41(ESV) — 16So Paul stood up, and motioning with his hand said: “Men of Israel and you who fear God, listen. 17The God of this people Israel chose our fathers and made the people great during their stay in the land of Egypt, and with uplifted arm he led them out of it. 18And for about forty years he put up with them in the wilderness. 19And after destroying seven nations in the land of Canaan, he gave them their land as an inheritance. 20All this took about 450 years. And after that he gave them judges until Samuel the prophet. 21Then they asked for a king, and God gave them Saul the son of Kish, a man of the tribe of Benjamin, for forty years. 22And when he had removed him, he raised up David to be their king, of whom he testified and said, ‘I have found in David the son of Jesse a man after my heart, who will do all my will.’ 23Of this man’s offspring God has brought to Israel a Savior, Jesus, as he promised. 24Before his coming, John had proclaimed a baptism of repentance to all the people of Israel. 25And as John was finishing his course, he said, ‘What do you suppose that I am? I am not he. No, but behold, after me one is coming, the sandals of whose feet I am not worthy to untie.’ 26“Brothers, sons of the family of Abraham, and those among you who fear God, to us has been sent the message of this salvation. 27For those who live in Jerusalem and their rulers, because they did not recognize him nor understand the utterances of the prophets, which are read every Sabbath, fulfilled them by condemning him. 28And though they found in him no guilt worthy of death, they asked Pilate to have him executed. 29And when they had carried out all that was written of him, they took him down from the tree and laid him in a tomb. 30But God raised him from the dead, 31and for many days he appeared to those who had come up with him from Galilee to Jerusalem, who are now his witnesses to the people. 32And we bring you the good news that what God promised to the fathers, 33this he has fulfilled to us their children by raising Jesus, as also it is written in the second Psalm, “ ‘You are my Son, today I have begotten you.’ 34And as for the fact that he raised him from the dead, no more to return to corruption, he has spoken in this way, “ ‘I will give you the holy and sure blessings of David.’ 35Therefore he says also in another psalm, “ ‘You will not let your Holy One see corruption.’ 36For David, after he had served the purpose of God in his own generation, fell asleep and was laid with his fathers and saw corruption, 37but he whom God raised up did not see corruption. 38Let it be known to you therefore, brothers, that through this man forgiveness of sins is proclaimed to you, 39and by him everyone who believes is freed from everything from which you could not be freed by the law of Moses. 40Beware, therefore, lest what is said in the Prophets should come about: 41“ ‘Look, you scoffers, be astounded and perish; for I am doing a work in your days, a work that you will not believe, even if one tells it to you.’ ”