The Greek word for "gospel" is evangellion, which roughly means "good news." And that's what all four of the gospels claim to share: the good news that God has sent Jesus into the world.
A gospel is different from a standard biography because it isn't just a record of a someone's life. Nope, a gospel has a clear purpose: it wants to make you believe. If you walk away after reading one of the gospels and your life hasn't been changed, then the gospels will hang their papery heads in sadness because they've failed to do their jobs.
The early Christians invented the idea of a gospel; up until then, nothing like it had ever been written. And their invention worked. Big time. In the last 2,000 years, the gospels have helped convince billions of people all over the world that Jesus is someone worth putting your faith in. (Source.)