"For I passed on to you as most important what I also received: that Christ died for our sins according to the Scriptures, that he was buried, that he was raised on the third day according to the Scriptures"
1 Corinthians 15:3-4, CSB
The gospel is the "good news" or the "grand announcement" of what God has done to save wicked and rebellious sinners through the person and work of His only Son, Jesus Christ.
In fulfillment of Old Testament prophecy, Jesus fulfilled the law of God perfectly, died on the cross to pay the penalty for the sins of His people, rose from the dead, and sat down at the right hand of God the Father. He now reigns over heaven and earth in all power and authority.
Any person who turns from their sin—acknowledging their helpless state before God—and believes in Jesus Christ as their Lord and Savior, is forgiven, freed, born again, declared righteous in God's sight, and promised eternal life with God.
"Therefore, we are ambassadors for Christ, since God is making his appeal through us. We plead on Christ’s behalf: “Be reconciled to God.” He made the one who did not know sin to be sin for us, so that in him we might become the righteousness of God."
2 Corinthians 5:20-21, CSB
"He himself bore our sins in his body on the tree; so that, having died to sins, we might live for righteousness. By his wounds you have been healed."
1 Peter 2:24, CSB
"...we know that a person is not justified by the works of the law but by faith in Jesus Christ, even we ourselves have believed in Christ Jesus. This was so that we might be justified by faith in Christ and not by the works of the law, because by the works of the law no human being will be justified."
Galatians 2:16, CSB
"Christ redeemed us from the curse of the law by becoming a curse for us, because it is written, Cursed is everyone who is hung on a tree. The purpose was that the blessing of Abraham would come to the Gentiles by Christ Jesus,
so that we could receive the promised Spirit through faith."
Galatians 3:13-14, CSB