I see the Gospel in its simplest definition as the "good news" Jesus preached - and that good news is centered on one concept:
I am a child of God.
I believe every truly unique doctrine within Mormonism, especially compared with Protestantism, derives from that concept, and I believe the zenith of that teaching of the good news is the Intercessory Prayer in John 17 - that all of us can become one with GOD, the Father (and Mother), and God, the Son, in the same way they are one. For me, everything else is variable detail - pieces of various puzzles that people use to attempt to fill the same framework or images on mosaics that we create in order to become a "true and living" replica of God.
I see the good news as the idea that such efforts to become godly replicas are not pointless or in vain - that these mortal caterpillar exteriors we inhabit really will be shed at some point, and we will emerge as the butterflies we were created to be. That requires faith, since, like caterpillars, we don't get to see the process of metamorphosis that makes us what we aren't currently (the cocoon process occurring for us after death - theologically phrased as "the spirit world" prior to becoming new, "resurrected" beings), but I believe it is that belief and hope in the unseen (that "faith") that is the foundation of the gospel Jesus preached - and it is the central "power of godliness" that is mentioned in JSH 1:19 that was denied within the Protestant creeds of Joseph's time (and still is now).