What is the Gospel?
It is the "good news", because it solves the problem we have as human beings. That problem is that God is holy and just, and we are not.
One day, we will be judged on our own merits (or lack of them) or on the goodness of another.
The good news of the Gospel is that Jesus lived a perfect life, obedient to God, not for Himself, but for His people. He offered Himself as a perfect sacrifice on the cross, to satisfy the justice and the righteousness of God. He has done for us what we couldn’t possibly do.
For God to forgive us is a very costly matter. It cost the sacrifice of His own Son. Christ died for us; He was raised so that we can be declared just. The Gospel is the message of who Jesus is and what He did. And it also tells us that the only way we can receive the benefit of Christ’s life, death and resurrection is by putting our faith and trust in Him - and in Him alone. If we do that, we’re declared just by God, we’re adopted into His family, all of the wrong things we’ve ever done are forgiven, and we have begun our journey to heaven.