I'm sure you're all aware that actually, it means 'good news', and has been used to mean any or all of the following: 1) The message Jesus proclaimed; 2) The message about Jesus; and 3) The records of Jesus' life, death, and a little bit after: 'The gospel according to...' But funnily enough, even biblical scholars go in for a bit of argy-bargy about the term 'gospel' (although none, to my knowledge, think that ghosts have anything to do with it). The big question is this: does the term come from the Jews or the Greeks? Answer, as always: probably a bit of both.
Isaiah talks a lot about God's message of salvation and promises to defeat Israel's enemies and put the world to rights. Both 'Gospel' and the title 'son of Man' occur in texts written by various Jewish sects of the time, often used to talk about some sort of messianic figure. When Jesus first appears, quoting Isaiah, it's clearly these ideas that he's invoking.
But the Greco-Roman world also had a lot of gospelly ideas knocking around, especially when talking about the Emperor and the imperial cult. The Greek word that the New Testament uses for gospel is 'euangelion', which means good news, but the Greeks and Romans tended to use 'euangelia' - the plural form (good newses?). In particular, it was good news when a new Emperor-to-be was born, and the Emperors tended to be portrayed as divine redeeming figures bringing peace to the whole world. Josephus, the David Starkey of his day, tells us that even the Jews saw Emperor Vespasian's ascension to the throne as gospelly good news. If you were a pagan in Jesus' day, then 'gospel' would have made you think of things to do with the Emperor. By referring to the message of Jesus as 'the good news', the gospel writers were being dangerously politically subversive - imagine talking about President Jesus, or Chairman Jesus, or Ayatollah Jesus, or Prime Minister Jesus (maybe not so much the last one. Poor Gordon). In the Empire, the Emperor is both King and God. His face is on all the coins, he's worshipped in the temples, he is The Man, and then this ragga bunch of gospel writers come along, saying, 'Good news! We have a new king, a new Emperor, and he will put the world to rights.' Controversial biscuits.
Photo credit: maistora on Flickr.