Gospel has a simple meaning, that of "good news" or "good story". If you were to ask Siri what the gospel is, the surprising answer is "the teaching or revelation of Christ". Even for Apple, the fact that the Gospel centers around Christ is inescapable.
Since Jesus is the center of the Gospel, the "Good News" He brings is the good news that He can forgive you of all of your sin. His death and resurrection is what was needed for your forgiveness. This forgiveness is not the default setting in your life, sin is. you have to come to God as a sinner and admit your sinfulness to Him.
Please watch the video below for more on what the Gospel is and what it can do in your life today.