It is impossible that Paul was saved under a different gospel to that which was established before Paul was saved.
No shift to another gospel was made even to this day.
LA
As within any school of thought (individuals within a same school will at times see some things differently) so within MAD.
The following is my understanding of this issue - other MADs may or may see some of these things differently...
Paul was not saveable prior to the gospel he was both saved under and then had committed unto him.
This here is WHY Paul was NOT saveable at that time...
Matthew 12:30 He that is not with me is against me; and he that gathereth not with me scattereth abroad. 12:31 Wherefore I say unto you, All manner of sin and blasphemy shall be forgiven unto men: but the blasphemy against the Holy Ghost shall not be forgiven unto men. 12:32 And whosoever speaketh a word against the Son of man, it shall be forgiven him: but whosoever speaketh against the Holy Ghost, it shall not be forgiven him, neither in this world, neither in the world to come.
Acts 7:51 Ye stiffnecked and uncircumcised in heart and ears, ye do always resist the Holy Ghost: as your fathers did, so do ye. 7:52 Which of the prophets have not your fathers persecuted? and they have slain them which shewed before of the coming of the Just One; of whom ye have been now the betrayers and murderers: 7:53 Who have received the law by the disposition of angels, and have not kept it.
Resistence of the Spirit was the unpardonable sin that Israel was warned would not be forgive them neither in that age; nor the one to come.
Just as the Spirit through Stephen reiterated to Israel; that to go against the Law of Moses the Spirit was now asserting to Israel the Law had prophesied that Jesus would be their prophesied Christ, was to continue in the UNcircumcision of their fathers against Moses.
Read it again, here, below...
Acts 7:51 Ye stiffnecked and uncircumcised in heart and ears, ye do always resist the Holy Ghost: as your fathers did, so do ye. 7:52 Which of the prophets have not your fathers persecuted? and they have slain them which shewed before of the coming of the Just One; of whom ye have been now the betrayers and murderers: 7:53 Who have received the law by the disposition of angels, and have not kept it.
What did Paul himself later say about that?
Romans 2:25 For circumcision verily profiteth, if thou keep the law: but if thou be a breaker of the law, thy circumcision is made uncircumcision.
He says that in Romans 2, as he heads towards the following conclusion against both Jew and Gentile...
Romans 3:9 What then? are we better than they? No, in no wise: for we have before proved both Jews and Gentiles, that they are all under sin; 3:10 As it is written, There is none righteous, no, not one: 3:11 There is none that understandeth, there is none that seeketh after God. 3:12 They are all gone out of the way, they are together become unprofitable; there is none that doeth good, no, not one.
Continuing on that, he goes on to relate that...
Romans 3:19 Now we know that what things soever the law saith, it saith to them who are under the law: that every mouth may be stopped, and all the world may become guilty before God. 3:20 Therefore by the deeds of the law there shall no flesh be justified in his sight: for by the law is the knowledge of sin. 3:21 But now the righteousness of God without the law is manifested, being witnessed by the law and the prophets; 3:22 Even the righteousness of God which is by faith of Jesus Christ unto all and upon all them that believe: for there is no difference: 3:23 For all have sinned, and come short of the glory of God; 3:24 Being justified freely by his grace through the redemption that is in Christ Jesus: 3:25 Whom God hath set forth to be a propitiation through faith in his blood, to declare his righteousness for the remission of sins that are past, through the forbearance of God; 3:26 To declare, I say, at this time his righteousness: that he might be just, and the justifier of him which believeth in Jesus.
Recall the Spirit's Words thru Stephen.
And Paul's words as to this grave issue he and his nation faced...but for Romans 3:21's "BUT NOW the righteousness of God WITHOUT the law..."
Paul's descriptiom of his spiritual UNcircumcision before that...
Acts 26:8 Why should it be thought a thing incredible with you, that God should raise the dead? 26:9 I verily thought with myself, that I ought to do many things contrary to the name of Jesus of Nazareth. 26:10 Which thing I also did in Jerusalem: and many of the saints did I shut up in prison, having received authority from the chief priests; and when they were put to death, I gave my voice against them. 26:11 And I punished them oft in every synagogue, and compelled them to blaspheme; and being exceedingly mad against them, I persecuted them even unto strange cities. 26:12 Whereupon as I went to Damascus with authority and commission from the chief priests, 26:13 At midday, O king, I saw in the way a light from heaven, above the brightness of the sun, shining round about me and them which journeyed with me. 26:14 And when we were all fallen to the earth, I heard a voice speaking unto me, and saying in the Hebrew tongue, Saul, Saul, why persecutest thou me? it is hard for thee to kick against the pricks.
Paul was in UNcircumcision spiritually.
Concluded under sin with his nation for his having blasphemed the Spirit.
In fact, their chief sinner against the LORD AND His Christ: for his having caused members of the Believing Remant of Israel to blaspheme the Spirit.
In other words, to recant their assertion that Jesus was the Christ or face Paul's wrath.
Paul was saved when God in His grace not only saved Paul in accordance with, but then commissioned him with - the gospel of the UNcircumcision.
Romans 3:22 Even the righteousness of God which is by faith of Jesus Christ unto all and upon all them that believe:
for there is no difference: 3:23 For all have sinned, and come short of the glory of God; 3:24 Being justified freely by his grace through the redemption that is in Christ Jesus: 3:25 Whom God hath set forth to be a propitiation through faith in his blood, to declare his righteousness for the remission of sins that are past, through the forbearance of God; 3:26
To declare, I say, at this time his righteousness: that he might be just, and the justifier of him which believeth in Jesus.
Much more could be said of this uniquely Pauline distinction.