The word Gospel literally means “good news.” It is the good news of salvation that God provided in the person and work of His Son, Jesus Christ.
More specifically to quote from scripture:
"that Christ died for our sins in accordance with the Scriptures, that he was buried, that he was raised on the third day in accordance with the Scriptures..."1 Corinthians 15:3-4
So the gospel is Christ's mission of salvation for the world, he's come to seek and save those who are lost while laying his life down as a ransom for many. To further understand the gospel we need to understand that Jesus was born of a virgin and that he was born under the law. He perfectly obeyed that law that in our disobedience he has taken our place.
He suffered the curse of our disobedience on the cross, and in doing so, Jesus who knew no sin, bore all the sins of all those who would ever believe in him as he hung upon the cross. Jesus propitiated for us before the righteous anger of God. All of this was accomplished by his substituting death in our place on the cross. Through him, he paid our debt in full.
The gospel is the free offer of salvation through the work of Christ to those who have no merit of their own by which to receive this gospel. It is offered as a free gift, and it can only be received with the empty hand of faith and to those whom He first calls.
Faith alone in Christ alone, offered by the grace of God alone. That is the gospel.