The word "gospel" is a direct translation into Old English of the Greek phrase euaggelion, which literally means good news. The Greek word was Latinized early on as evangelium; "gospel" is still evangelium in German and évangile in French. Even in English, the authors of the gospels are called the evangelists and missionary work is often referred to as evangelical.

The gospel of Mark introduces itself as "the good news (euaggelion) of Jesus Christ, son of God," which is how the story of Jesus' life came to be called by that name. Note that Mark's gospel is the only one that explicitly calls itself an euaggelion. Luke's book refers to itself as an "orderly account," while John and Matthew don't provide a word for what they are.

Since then, a few different but related meanings for the word "gospel" have developed.

• It could refer to one of the four accounts of Jesus' life in the New Testament: Matthew, Mark, Luke, and John.
• It could refer to a kind of narrative, specifically the kind that is found in the New Testament gospels. The first four books in the New Testament are an unusual combination of biography, historical account, theological commentary, and adventure-story, and soon the word "gospel" was used to describe any book that contained these narrative elements. Many such books exist beyond the four that made it into the canon. The Infancy Gospel of Thomas and the Proto-Gospel of James, both written during the second century, are good examples of stories that try very hard to imitate the style of the four canonical gospels.
  (Ironically, the most famous apocryphal gospel, namely the Gospel of Thomas, is not actually a good representative of the gospel genre, since it is simply a list of sayings without any connecting plot or dialogue.)
  It should be noted that some modern authors try to write gospels too, with varying levels of success. In 1997, James P. Carse wrote a book called The Gospel of the Beloved Disciple , which was written from the point of view of a woman.
  
• The word "gospel" could also refer to the content or message of the narratives in the New Testament, which is to say, the assurance of salvation promised by Christianity. This use of the word is typified by Matthew 24:14, in which Jesus predicts a time when the "good news (euaggelion) of the kingdom will be preached throughout the whole world." It seems to be this sense in which Paul uses the word in his letters, since he had no way of knowing about the specific documents that we know as Matthew, Mark, Luke, and John.
• Finally, by extension, it could be used as a way of talking about any book that has complete authority over someone's life. No doubt many of you have frequently uttered sentences like "I take everything I read on E2 as gospel truth."

Gos"pel (?), n. [OE. gospel, godspel, AS. godspell; god God + spell story, tale. See God, and Spell, v.]

1.

Glad tidings; especially, the good news concerning Christ, the Kingdom of God, and salvation.

And Jesus went about all Galilee, teaching in their synagogues, and preaching the gospel of the kingdom.
Matt. iv. 23.

The steadfast belief of the promises of the gospel.
Bentley.

⇒ It is probable that gospel is from. OE. godspel, God story, the narrative concerning God; but it was early confused with god spell, good story, good tidings, and was so used by the translators of the Authorized version of Scripture. This use has been retained in most cases in the Revised Version.

Thus the literal sense [of gospel] is the "narrative of God," i. e., the life of Christ.
Skeat.

2.

One of the four narratives of the life and death of Jesus Christ, written by Matthew, Mark, Luke, and John.

3.

A selection from one of the gospels, for use in a religious service; as, the gospel for the day.

4.

Any system of religious doctrine; sometimes, any system of political doctrine or social philosophy; as, this political gospel.

Burke.

5.

Anything propounded or accepted as infallibly true; as, they took his words for gospel.

If any one thinks this expression hyperbolical, I shall only ask him to read Oedipus, instead of taking the traditional witticisms about Lee for gospel.
Saintsbury.

© Webster 1913.

Gos"pel, a.

Accordant with, or relating to, the gospel; evangelical; as, gospel righteousness.

Bp. Warburton.

© Webster 1913.

Gos"pel, v. t.

To instruct in the gospel.

Shak.

© Webster 1913.