I enjoyed reading the following excerpt from Tim Grass' recent biography on F. F. Bruce.
So for Bruce, an evangelical was one who embraced the grace of God offered in the gospel. But what was the gospel? He answered this most fully in an early Rylands lecture, ‘When Is a Gospel Not a Gospel?’ In it he differentiated between true and false gospels, as Paul on occasion had to do. On the one hand he distinguished the reliable accounts of the life of Jesus found in the canonical gospels from non-canonical documents bearing the title 'Gospel' such as the Gospel of Thomas, which some argued offered alternative sources for understanding the life and teaching of Jesus. On the other hand he argued that a distinction between true and false gospels could be made with reference to the way in which hearers were said to benefit from the events proclaimed in them: 'The gospel which was no gospel probably did not differ from Paul's gospel with regard to the basic recital of saving events. Where it differed was with regard to the terms on which the benefits accruing from the saving events might be enjoyed.'" In Bruce's understanding of the New Testament evidence, a gospel was not a gospel when (i) it became detached from the Jesus of history; (ii) it gave little or no place to the passion of Christ; (iii) it exalted human achievement rather than the grace of God; (iv) it added other conditions to what God required; and (v) it treated righteousness and purity as things which those who were spiritual had outgrown. It was a gospel when (i) it maintained contact with the Jesus of history; (ii) it embraced 'the offence of the cross'; (iii) it extended grace to human beings for their acceptance by faith; (iv) it relied on the Holy Spirit to make its preaching effective to hearers; and (v) it issued in a righteous life sustained and directed by the love of God.
Tim Grass, F. F. Bruce: A Life (Grand Rapids: Eerdmans, 2012), 154-55.
Grass is drawing from F. F. Bruce, "When Is a Gospel Not a Gospel?" BJRL 45 (1962-3): 319-39.