The word "gospel" means "good news." What is the gospel of Jesus Christ? A good way to summarize the gospel is to biblically un-pack the words God, Man, Christ, and Response.

God

God is the Creator of all things. He is perfectly holy, worthy of all worsip, and will punish sin.
Genesis 1:1
1 John 1:5
Revelation 4:11
Romans 2:5-8

Man

All people, though created good, have become sinful by nature. From birth, all people are alienated from God, hostile to God, and subject to the wrath of God.
Genesis 1:26-28
Psalm 51:5
Romans 3:23
Ephesians 2:1-3

Christ

Jesus Christ, who is fully God and fully man, lived a sinless life, died on the cross to bear God’s wrath in the place of all who would believe in him, and rose from the grave in order to give his people eternal life.
1 Timothy 2:5
Hebrews 7:26
Romans 3:21-26
2 Corinthians 5:21
1 Corinthians 15:20-22

Response

God calls everyone everywhere to repent of their sins and trust in Christ in order to be saved.
Mark 1:15
Acts 20:21
Romans 10:9-10