Gospel (Gr. εὐαγγέλιον (euangélion, Strong's G2098) is roughly translated as "good news", "good tidings" or "good message". It is strongly tied to Strong's G2097, εὐαγγελίζω or euangelízō, from which we get the English word "evangelize".
This should lead one to question, "Good news about what?" There is a mass of confusion on this matter, and many preachers and churches will tell you different things. In many cases, their message is that if you rotely say something called "The Sinner's Prayer", then you are "saved", and all without any explanation of what exactly you are supposed to be saved from. The truth is that many mainstream Christian teachers focus on one, maybe two, aspects of the real good news and ignore the rest, thus presenting a very incomplete message of The Gospel.
One conflating factor in all of this confusion is the fact that the first four books of the New Testament are called "The Gospels" because they give an account of the birth, life and death of Jesus Christ as well as His teaching. That makes many believe that The Gospel is the story of the life and death of Jesus on the cross for our sins and never take it to its full meaning, which was expounded upon in Jesus' own teaching.