The Gospel is the evangel, the good news, closely related to the kerygma, the message and proclamation of the God of heaven and earth anchored in redemptive events of human history - notably the loving, sacrificial death; victorious, bodily resurrection; and glorious, conquering second coming of Jesus.
He then goes after some distressingly common distortions of the Gospel in certain Protestant circles today:
The Gospel is not a free ticket to a halo-and-harp-studded heaven for rebels who want a little eternal life insurance. The Gospel is not merely a Get-Out-of-Hell-Free card. The Gospel is the trumpet blast of the King.
He rightly distinguishes between technically correct soteriological belief and knowing-in-Whom-we-have-believed-and-trusted-for-salvation:
They need not know "theology" to be saved; they do need to know that Jesus and His redemptive work is their only hope and that in trusting Him, they are abandoning themselves to Him.
See his entire article, What is the Gospel?, from the Reformed Catholicism blog.
Here are my own related papers. The first dates originally from 1982. I recently added some commentary designed to show how Catholics would express things differently, or add a few points to the usual evangelical presentation of the Gospel:
Good News: An Evangelical / Catholic Presentation of the Gospel Message
What is the Gospel?
The Gospel, as Preached by the First Christians