Dictionary.com

—noun

1. the teachings of Jesus and the apostles; the Christian revelation.
2. the story of Christ’s life and teachings, especially as contained in the first four books of the New Testament, namely Matthew, Mark, Luke, and John.
3. (usually initial capital letter) any of these four books.
4. something regarded as true and implicitly believed: to take his report for gospel.
5. a doctrine regarded as of prime importance: political gospel.
6. glad tidings, especially concerning salvation and the kingdom of God as announced to the world by Christ.
7. (often initial capital letter) Ecclesiastical. anextract from one of the four Gospels, forming part of the Eucharistic service in certain churches.
8. gospel music.

—adjective

9. of, relating to, or proclaiming the gospel or its teachings: a gospel preacher.
10. in accordance with the gospel; evangelical.
11. of or relating to gospel music: a gospel singer.

Origin: before 950; Middle English go(d)spell, Old English gōdspell (see good, spell²); translation of Greek euangélion good news; see evangel¹

—Related forms
non·gos·pel, adjective

Discussion/Explanation

As you can see, depending upon whether the word is singular, plural, or different context, the meaning of the term seems to vary.

The term by itself means as you see in the origins section – good news. The good news of the Scriptures would be the first 4 books of the New Testament (NT) which include Matthew, Mark, Luke, and John.

Each of these books is named after their writers. Each tells the story of Christ. Matthew and Mark are the most alike. Luke has much in common with the first two, but it includes the perspective of a physician as Luke was what we call today a doctor.

These first 3 books of the Gospels are often called the Synoptic Gospels because of their commonalities.

John is unique. This becomes immediately obvious upon reading just the first few verses of John 1. You see an immediate emphasis on the deity of Christ and this continues throughout the book. As such, the story of Christ as God is what you read in John resulting in not all of the same events as in the first 3 being told.