To many evangelicals, especially in the Reformed tradition, what is meant by the Gospel is simply the Doctrine of Justification. In fact, if one questions the very narrow understanding of Justification put forth by some Reformed theologians one runs the risk of being called a heretic (at best) or (at worst) being dangerously close to denying the Gospel altogether. Just ask N.T. Wright! Most Christians probably could not even give a lucid (much less comprehensive) biblical definition of Justification, and yet many theologians equate this doctrine with the Gospel itself.
What is the Good News (Gospel) that is to be believed for salvation? Is the Gospel "justification by faith"? Is the Gospel "penal substitution"? Is the Gospel "do unto others..."? Is the Gospel "feed the poor"? Is the Gospel "believe on the Lord Jesus..."?
In the link below, the writer helpfully distinguishes between the Gospel and the implications of the Gospel.
What is the Gospel? Jesus is here! And He has ushered in the Kingdom of God!
Now believe it...and live it!