The gospel is the good news that God has made a way for sinners to be forgiven and go to heaven. Of course, the "good news" only makes sense when we understand the bad news that we are all sinners and deserving of judgment. The good news of the gospel is that while we cannot save ourselves, we do not need to try to do so. Our salvation is a free gift made possible through the work of Jesus Christ on the cross.
A helpful explanation has been written which explains the gospel by considering four words: God, man, Jesus, response.