I’ve been reading an old article by F.F. Bruce, “When Is a Gospel Not a Gospel?” BJRL 4 (1963): 319-39. Bruce concludes with this:
To sum up, then, we may say that, according to the general consensus of the New Testament teaching, a gospel is not a gospel when:
1. It is detached from the Jesus of history;
2. It gives little or no place to the passion;
3. It exalts human achievement in the place of the grace of God;
4. It adds other conditions to the one which God has declared acceptable (even if those additions be things good and desirable in themselves); or
5. It treats righteousness and purity as things which the truly spiritual man has outstripped.
On the other hand, a gospel is a gospel when:
1. It maintains contact with the Jesus of history, affirming that “this same Jesus” who came in the flesh and died is the vindicated and exalted Lord;
2. It embraces and proclaims “the stumbling block of the cross”;
3. It extends the grace of God to men for their acceptance by faith;
4. It relies upon the power of the Spirit to make it effective in those who ear it; and
5. It issues in a life of righteousness and purity which is sustained and directed by the love of God.
I think is pretty good, accept there is no concrete connection to the narrative of redemptive-history, resurrection needs a mention, and for christology we have the mere fact of Jesus’ earthly existence rather than linking his messianic career and passion together.