The gospel is “The Good News” that God became man in Jesus Christ. That He lived the life we should have lived. The Lord Jesus died the death we should have died, because of our sin. He freely went to the cross in our place. Three days later, The Lord Jesus rose from the dead. He proved that he is the Son of God. The Lord Jesus offers the gift of forgiveness of sin, and eternal life through his blood atonement. This is a free gift to all, who by faith, repent of their sins, and receive Jesus Christ as both Lord and Savior.
For more information look to the book of Romans in the Bible.