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
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:
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.
There was a gospel before the Cross.
It too was based on faith. Paul identifies it in Romans 2 and 3, and again in Romans 9 and Romans 10, and also in Galatians 3 - that righteousness which was of the Law by faith he relates Israel was under until Christ.
In John 5, for example, the Lord condemns those of Israel for not believing Moses that He was the One Moses had written of.
John 5:
43. I am come in my Father's name, and ye receive me not: if another shall come in his own name, him ye will receive.
44. How can ye believe, which receive honour one of another, and seek not the honour that cometh from God only?
45. Do not think that I will accuse you to the Father: there is one that accuseth you, even Moses, in whom ye trust.
46. For had ye believed Moses, ye would have believed me; for he wrote of me.
47. But if ye believe not his writings, how shall ye believe my words?
Obviously, this faith in Him was not that He died for sin, for He was sometime off before the Cross.
It is clear in Matthew 23, in light of Romans 2, that Israel's religious leaders distorted this righteousness which was of the Law by faith into the error that simply keeping the Law would meet God's standard.
As a result, they were so used to keeping the Law not by faith, rather; simply as a means of proving their righteousness before men, that when He "of whom Moses in the law, and the prophets, did write, Jesus of Nazareth, the son of Joseph," John 1:45, finally showed up, their response to Him was that...
"...Israel, which followed after the law of righteousness, hath not attained to the law of righteousness. Wherefore? Because they sought it not by faith, but as it were by the works of the law. For they stumbled at that stumblingstone; As it is written, Behold, I lay in Sion a stumblingstone and rock of offence: and whosoever believeth on him shall not be ashamed" Rom. 9:31-33.
Throughout the Lord's ministry to that nation, he chastises them for doing this and that before men; that men might glory in them.
By the time of Galatians, you have that distortion - of the law for righteousness by faith absent of faith - now coming against that "righteousness of God without the Law" - the issue of being "justified freely by his grace through the redemption that is in Christ Jesus" Rom. 3.
What this all boils down to is...
1] One actual gospel had been Messianic Judaism, see John 1.
2] But that Israel's leaders had distorted into "works of the law, as pleasing men, not God, " John 8; Rom. 2.
And then you have another...
3] Actual gospel also - "now the righteousness of God without the Law... by faith" Rom. 3.
# 3 was distorted by the Judaizers into # 2, which is why it was another [where their were two legitimate ones] which was not another at all.
1 and 3 were both legitimate.
You'll note I did not base this on the Greek, or on this or that word, mood, sense, and all that, rather; on Scripture's own overall narrative.
Why?
Because that is what 2 Timothy 3:16-17 asserts the Scripture is able to bring to the table as to any and all issues; all on its own, and then some!