Recently, I was licensed by my church (Now, I can officially "marry and bury"). In the process, I had to think hard about and formulate in a paragraph an answer to the question: "What is the Gospel?" Greg Gilbert has even written a book on it.
A happy God eternally existing in three persons out of his good pleasure creates time and space and fills both with created things. Man, his image-bearer and apex of his creation rebels against his Creator. In his good pleasure, God chooses to redeem his fallen creation. The Gospel particularly is the great news of God's coming promise in the form of Jesus, the Son of God to incarnate, obey the Law's demands perfectly, suffer for sinners to atone for their guilt and satisfy God's rightful wrath. Furthermore, it is the death, the burial and the bodily resurrection of Jesus from the dead. It is further good news that this Jesus' death and life (his righteousness) are graciously "imputed" to a believer's account. It is good news that the Holy Spirit is given as a sign and seal of that redemption. The gospel at the end of the day is the hope and joy of eternal life. Amen!
Also, I got the chance to answer this question in a video format: