The gospel is the good news of God's plan and work to save sinners from His wrath and bring them into a right relationship with Himself through His Son, Jesus Christ — through his life, death, resurrection, and ascension. This offer of salvation is for all who will turn from sin and trust fully in Jesus.
For a thorough explanation of the message of the gospel, as well as how you should respond to it, take a look at Two Ways to Live.