The entire piece is worth a read (http://www.bibleinterp.com/articles/mason3.shtml)...the singular neuter form euangelion was extremely rare before the NT.... The novelty of Christian usage is unmistakable. The small New Testament library includes 76 occurrences (including the long ending of Mark, at 16:15) of the neuter singular, and nearly all of these (72) include the definite article: to euangelion. Something unusual is happening here, which calls for an explanation. At about the time of the latest New Testament writings (ca. 110 CE), Ignatius of Antioch used the word group 24 times in his very small group of short letters, and 21 of these cases have the distinctive form to euangelion. After Ignatius, Christian authors of the second century continued to use the word group eagerly. Among the Greek fathers Origen, Eusebius, Cyril, Theodoret, John Chrysostom, and the Gregories use euangeli- words hundreds of times each. The pattern is clear. This language is markedly favored by early Christians from the second century and hardly used by anyone else. It was the NT collection that established this usage. The early Christians were not using common language for “good news.”
That in itself is perhaps not surprising, but now we come to the data that really upset the applecart. Within the NT collection, distribution of to euangelion is in no way proportionate. The genuine and disputed letters of Paul, although they occupy somewhat less than a quarter of the NT (about 32,445 of 138,000 words), account for 60 of the 76 occurrences of the neuter singular. Now, Paul’s letters are the earliest Christian writings to have survived, belonging to the first generation after Christ (roughly 30 to 65 CE). The Gospels belong to the next generation, from 35 to 100. Of the non-Pauline material in the NT, Mark is the heaviest user with 8 occurrences (including the long ending), all of these with the article. Thus, Paul (including pseudo-Paul) and Mark together account for fully 67 of 72 occurrences of to euangelion. By contrast Matthew, though most scholars think that its author used Mark as a source, taking over more than 90% of the earlier text and adding about 50%, has only 4 occurrences of this noun. Most surprisingly, although it also used Mark as a source, Luke omits the noun altogether and Acts has it only twice, though this “double work” accounts for nearly half (25) of the NT’s 54 occurrences of the cognate verb euangelizō. John has no trace of the word group in any form, and the hypothetical sayings Gospel Q along with the structurally similar Thomas lack the noun. Hebrews also omits the noun, though it has the verb twice.
In short, then, a triple movement needs explaining: first, why Paul and Mark seized upon the hitherto unused form to euangelion so energetically and programmatically, almost always without qualification; second, why all of the next-generation texts except for Mark drew back and avoided the term (or qualified it if used); finally, why from the third generation onward does to euangelion become a fundamental term of shared Christian discourse?
Mason goes on to examine some of the features of Paul's proprietary usage of the term. Perhaps his most important observation is that Paul's gospel is not a point of unity, but a point of difference and distinction, between him and other Christian groups at the time, including the James/Peter circle, and the recipients of his Epistle to the Romans. Paul preaches "his gospel of Jesus Christ." Of all the others who write about Jesus--Matthew, Luke, John, Q, Thomas--only Mark shares both Paul's distinctive usage of to euangelion, along with distinctive Pauline themes, including: imminent apocalypse, openness to gentiles, relaxation of purity laws, and a low regard for Peter and James.
The question why the 2nd century church came around to adopt to euangelion as one of the central normative concepts of Christianity, although earlier authors were wary of its Pauline flavor, seems essential if we want to understand the invention of the gospel genre. Is it not the case that the inscription to euangelion kata ___ was simply appended at some late stage by the redactors to the 4 canonical "gospels"? Is this an instance of co-opting Paul's most definitive term in order to distract from his originality in conceiving the Christian idea?