Gospel means good news. So whats the good news? In another way you can call it the cure. The only way a person is going to appreciate the cure or the good news is if they understand that they have a disease or if they think that they actually want to hear good news. Sin is the disease. The Gospel is the cure. God's wrath abides on you but Jesus came to give you a way out. Not only that but He gives us a way in. A way in to the Father.
When Jesus was saying "repent and believe in the gospel" He was basically saying "Turn from you sin and believe in the Messiah and what He is going to do".
Jesus is what makes the gospel good. It is in the telling of His story that brings glory to God.