For me shorthand, or the short answer for "what is the gospel" has always been that the gospel is the death, burial and resurrection of Jesus Christ according to the Scriptures. That is a true, but all 15 observations below indicate it is much more complicated than that.
1. The gospel, in some sense, begins with John the Baptist (Mark 1:1). But it was preached to Abraham (Galatians 3:8), and can also be called everlasting (Revelation 14:6).
2. The gospel can include a memorial story of a woman anointing Jesus with oil (Matthew 26:13).
3. The gospel can be preached (Mark 16:15; I Corinthians 1:17; 15:1).
4. The gospel belongs to the kingdom of God (Matthew 24:14; Mark 1:14-15).
5. The gospel includes and/or belongs to the grace of God (Acts 20:24).
6. The gospel is the power of God unto salvation (Acts 20:24 ).
7. The gospel can be obeyed or disobeyed. (Romans 10:16; II Thessalonians 1:8; I Peter 4:17).
8. The gospel can be received and "stood in" (1 Corinthians 15:1)
9. The gospel is the death, burial and resurrection of Jesus Christ according to the Scriptures (I Corinthians 15:1-4).
10. The gospel can be hid to them that are lost (II Corinthians 4:3-4).
11. The gospel is unique and unalterable (Galatians 1:6-8)
12. The gospel is the word of the truth (Colossians 1:5).
13. God by the gospel calls us to the obtaining of the glory of our Lord Jesus Christ (II Thessalonians 2:14).
14. Jesus Christ brings life and immortality to light through the gospel (2 Timothy 1:10).
15. The eternal word of the Lord is preached unto us by the gospel (I Peter 1:25).