In The Genius of Luther's Theology by Kolb and Arand, the authors suggest that we should primarily understand 'gospel' in terms of promise:
'While "good news" provides an etymological translation of euangelion (gospel), it runs the risk of being viewed primarily as information about yesterday's events. Like a newspaper, it deals with past events, in this case, the past events of Jesus's earthly life. But when that happens, faith can be seen as little more than a form of intellectual activity, what the reformers criticized as historical faith. Such faith cannot by itself save since even the devils believe that Jesus died and rose. By itself, the biography of Jesus is not yet gospel. It becomes gospel when it grasps the sinner with the promise that Christ lived, died and rose "for you!" and "for me!" and "for us!"' (p.42).
I am not sure about the idea that the NT is only gospel once it 'grabs' someone - I think in itself it is a promise from God. However, I do think this emphasis on promise is helpful. Later on the authors note that:
'A promise without faith accomplishes nothing, But faith without a promise has nothing to which it can cling. For Luther and Melanchthon, the promise of new life in Christ and faith in that same promise were corollaries. A promise by its very nature seeks a response. [...] At the same time, Luther stressed that trusting the promise is not an accomplishment that we can claim for ourselves. On the contrary, faith is the work and gift of God, who justifies a person by giving faith to that one. To that end the promise of the gospel itself creates and sustains that which it seeks: faith.' (p.45)
I like this idea of relating the gospel and faith as promise and response. It helps tie faith to the gospel, rather than have it as a free-floating virtue. Modern culture loves the idea of 'faith' - one (obscure) example that springs to mind is the Prince of Egypt cartoon that (more or less) faithfully renders the story of Moses and the Exodus. However, the big lesson is that you have to have faith - but faith's object is never proclaimed. However, if we think of faith as response, to say 'you just have to respond' immediately begs the question 'to what?!'