As Wright says, these heralds didn't ride into town and say, "In case you are interested in offering your devotion to someone, you may wish to try Caesar, who has recently become the Emperor of Rome. He would be delighted if you should wish to follow him." Instead, it would have been more like this, "We bring you the good news ('gospel') of Rome that Caesar Augustus is now our beloved Emperor, and demands your allegiance and taxes! On your knees!" In short, these heralds were declaring that Caesar was Lord!
The idea of 'good news' was certainly not just a Roman thing, however. The Jewish prophet Isaiah had spoken centuries ago about 'good tidings' for the poor, etc. (ch. 52 & 61 and other places). In fact, when Isaiah was translated into Greek (in the Septuagint), they used the same word ('euangelion') in these places! Indeed, the word 'gospel' had a very different usage when the New Testament was written!
Believing the 'Gospel' in the first century came complete with side-effects, and it wasn't simply that you belonged to a club that you didn't before. If it was the Gospel of Caesar, the side-effect was that you would swear allegiance to him as Lord - lived out by paying taxes and obedience to the Roman system. Whether you were an orator, civic benefactor, patron, client, land-owner, peasant or slave, obedience meant knowing your place and not rocking the boat.
With the coming of Jesus, the word Gospel took on new meaning - as did the side-efffects that went with believing it. For Jews, believing the Gospel of Jesus meant that the 'good news' of Isaiah had never been announced like it had been with Jesus. In the Roman world, however, believing the Gospel of Jesus was hazardous for your health! Believing that Jesus was Lord meant believing that Caesar was not! It meant believing that the 'good news' of Jesus made the 'good news' of Rome look like a cheap scam. It meant no longer living according to a system which really only served an elite few at the top, but rather living according to the character of a Lord, who is nothing at all like Caesar.
What in the world does this mean for us today? Possibly more than we care to know. I think it means that the Gospel of Jesus has little to do with an invitation that I accept (as if it were about me), and everything to do with an announcement that is true. Jesus really IS Lord. My life needs to give voice to that, and simply saying so won't do. Simply associating with others that say so (or not associating with those that don't) won't do either. Our spending habits, dreams ('american' or otherwise), time, money, standard of living, and much more - they all must bow the knee to the fact that Jesus is Lord. Announcing this is our calling. It will definitely require our words, but equally (or more) so, it will require our lives.