I'm confused, if there is only one gospel why do so many people says the four gospels, Matthew, Mark, Luke, and John?
The gospel is the good new preached concerning Jesus Christ and salvation. All of the New Testament is the gospel in this sense. See The Message Preached to see that the Faith, the Truth, the Doctrine, the Way, the Word and the Gospel are all speaking of the same thing.
The first four books are the gospel accounts. They are titled "The Gospel of Matthew," The Gospel of Mark," "The Gospel of Luke," and "The Gospel of John." The four are referred to as the gospels (notice the plural) and are treated in a similar way to first five books of the Old Testament being called the Law, even though the entire Old Testament is also called the Law.
The four gospel accounts form the core of the gospel and, though they are four in number, they don't teach a different message so they are one in what is communicated.