The word "gospel" means "good news". In the Christian sense, the gospel is the good news that God has sent his son, Jesus, to provide a way for people, who are separated from Him, to be made right with Him again. The gospel is presented in the Bible, as each book contributes to the larger storyline - that God has been working throughout history to restore His Creation and to proclaim His glory.
If you would like more information about the gospel, we invite you to read the this article from Crossway.org or you can watch this short Video.