The word gospel means infalliable truth.
"The gospel" refers to the good news concerning Jesus Christ.
"For God so loved the world, that He gave His only begotten Son, that whoever believes in Him shall not perish, but have eternal life." (John 3:16 NASB)
While we use the term "The Gospels" to refer to the first four books of the New Testament, the term "the gospel" can refer to the entire Bible, because every book in the Bible proclaims the good news concerning Jesus Christ and is infalliable truth.