The word "gospel" means "good news." It is the good news that Jesus Christ, the eternal Son of God, died on the cross for our sins and was raised from the dead. It's the good news that anyone who will believe on Christ Jesus may be forgiven of their sins. It is the good news of salvation and eternal life through Jesus.
The message is beautifully conveyed in John 3:16 where the Bible says, "For God so loved the world that He gave His only begotten Son that whosoever believes on Him shall not perish but shall have everlasting life." These famous words were spoken by Jesus Christ and they represent the good news of God's salvation.