The gospel is the good news of the life, death, and resurrection of the God-man, Jesus Christ. In his book What Is the Gospel?, R.C. Sproul wrote:

“The gospel is called the ‘good news’ because it addresses the most serious problem that you and I have as human beings, and that problem is simply this: God is holy and He is just, and I’m not. And at the end of my life, I’m going to stand before a just and holy God, and I’ll be judged. And I’ll be judged either on the basis of my own righteousness–or lack of it–or the righteousness of another. The good news of the gospel is that Jesus lived a life of perfect righteousness, of perfect obedience to God, not for His own well being but for His people. He has done for me what I couldn’t possibly do for myself. But not only has He lived that life of perfect obedience, He offered Himself as a perfect sacrifice to satisfy the justice and the righteousness of God.” †

Jesus Christ perfectly obeyed the law of God and died on the Cross as our substitute and as the payment for the penalty of our sins. And then Jesus rose from the dead on the third day.

“By his resurrection his righteousness is confirmed. The resurrection of Jesus confirms His victory over death and hell. Christ’s resurrection is the death of death. It signals the vindication of His claim to deity. It corroborates every word He uttered. It tells us that the gospel is true and trustworthy.” †

Learn more from R.C. Sproul:
Why Is the Gospel Called “Good News”?

† Quotes from What Is the Gospel? by R.C. Sproul.