The word "gospel" means "Good News" of Jesus.
The Most High God created humanity to worship Him. Humans chose instead to worship lesser things, which seperated them from God, leaving them spiritually dead and without hope. In his mercy, God provided a way for humans to be restored to His original design. He sent His Son, Jesus, to live a perfect life on earth, die a torturous death in our place, and come back to life in a supernatural demonstration of his power and love. Those who orient their lives around Jesus are brought back into a right relationship with God, who then sends us into the world on His mission of redemption.
This is the gospel.