There is really no need to wonder if there is one gospel or more than one. The Apostle Paul already answered this question
6 I am astonished that you are so quickly deserting him who called you in the grace of Christ and are turning to a
different gospel—
7 not that there is
another one, but there are some who trouble you and want to distort the gospel of Christ. (
Galatians 1:6-7 ESV)
The word “different” is
heteros which means “
different in kind or nature.”
http://biblehub.com/interlinear/galatians/1-6.htm
In verse 6, the teaching of the Judaizers was called a
heteros “gospel” because it was utterly unlike, and antithetical to the gospel of grace that they were departing from. So that the Galatians would not think there were multiple (legitimate) gospels Paul adds in verse 7
“not that there is
ANOTHER one.”
The word he uses this time is
allos which means “
another of a like or similar kind.”
A comparison of the words heteros and allos can be found in Strong’s Concordance
2087
héteros –
another (of a different kind). 2087 /héteros ("another but distinct in kind") stands in contrast to 243 /
állos ("
another of the same kind"). 2087 /héteros ("another of a different quality") emphasizes it is qualitatively different from its counterpart (comparison).
http://biblehub.com/greek/2087.htm
Paul said that no other gospel was
the same as or similar to the one he preached. However, if the dual gospel hypothesis were correct there WOULD have been
another gospel similar to his – the so called “Jewish Gospel” This particular gospel if it had existed would have been taught by Jesus Himself in person to the Twelve Apostles even as Jesus supposedly revealed Paul's gospel to him through supernatural revelation.
“Peter’s gospel” though different was apparently powerful to save men just as Paul’s. Had a saving “Jewish Gospel” existed it would not have been called
heterodox (different and therefore false). It would have been another (allos) gospel of the same or similar nature to Paul’s. In
Galatians 1:7, however, Paul flatly denies that there is any other gospel AT ALL.
Is this just a grammatical illusion? Would someone who lived at that time, who grew up speaking and reading the Greek of the early centuries, have understood these verses this way? At this point we might wish we could use Dr. Who’s Tardis to go back in time and discuss it with someone who lived then.
Fortunately we can hear from people of that era. Several men - one from the Third Century and the other from the Second commented on these very verses in their writings. The first was a man of God, John Chysostom who is regarded by many as the greatest Bible expositor of the Third and Fourth Centuries. The second was Marcion the arch-heretic of the Second Century. Despite his heterodox views, Marcion had access to the very earliest editions of the NT and, because he spoke Koine Greek he could understand the meaning of the words in Galatians without aid of lexicons or translations.
When Marcion read
Galatians 1:6-7 it was obvious to him that Paul was saying there was no other gospel AT ALL. Therefore, he reasoned that Paul’s revelatory Gospel of Grace superseded the teaching of all the other Apostles and even what Jesus Himself had taught during His earthly ministry. Marcion was so convinced of his insight that he produced his own edition of the NT minus everything but the Gospel of Luke and the writings of Paul (without the pastoral epistles)
Here is what Chrysostom wrote in response to Marcion:
Galatians 1:7 “Which is not another Gospel.”
And justly, for there is not another. Nevertheless the Marcionites are misled by this phrase, as diseased persons are injured even by healthy food, for they have seized upon it, and exclaim, “So Paul himself has declared there is no other Gospel.” For they do not allow all the Evangelists, but one only, and him mutilated and confused according to their pleasure.* Their explanation of the words, “according to my Gospel and the preaching of Jesus Christ,” (Rom. xvi. 25.)** is sufficiently ridiculous; nevertheless, for the sake of those who are easily seduced, it is necessary to refute it. We assert, therefore, that, although a thousand Gospels were written, if the contents of all were the same, they would still be one, and their unity no wise infringed by the number of writers.
*the Book of Luke
** the Marcionites thought that when Paul used the term "my gospel" which he did once, it meant that it had come to and through him only
John Chrysostom Homilies on Galatians Chapter II.
http://www.ccel.org/ccel/schaff/npnf113.iii.iii.ii.html
Chysostom agreed that the phrase “
which is not another gospel” meant
there was no other gospel at all. However, unlike Marcion, Chrysostom in line with the Church Fathers of the preceding three centuries believed in the essential unity of the NT message as it was expressed in Matthew, Mark, Luke, John and in the writings of the other Apostles (including Paul). To the Early Church all of it was equally scripture, profitable for doctrine, reproof, instruction in righteousness. The Marcionite’s belief that Paul had a different gospel and that only his writings were mandatory and applicable to believers was universally rejected as aberrant.