If your OT-based idea is true then Paul's gospel wasn't a mystery (Ephesians 6:19), was it? Unless there were other things Paul taught in his gospel that nobody else knew about... like parts of 1 Corinthians 15:5-8. If I were you, I'd have to look in Luke's writings for this "scripture" Paul references.
Personally, from the WAY that he writes, I think that Paul considered ALL of his teachings to be his gospel to the gentiles.
In several places the NT Scriptures speak of a mystery.
Ephesians 3:1-7
For this reason I, Paul, the prisoner of Christ Jesus for you Gentiles — 2 if indeed you have heard of the dispensation of the grace of God which was given to me for you, 3 how that by revelation He made known to me the mystery (as I have briefly written already, 4 by which, when you read, you may understand my knowledge in the mystery of Christ), 5 which in other ages was not made known to the sons of men, as it has now been revealed by the Spirit to His holy apostles and prophets: 6 that the Gentiles should be fellow heirs, of the same body, and partakers of His promise in Christ through the gospel, NKJV
A “mystery” according to Webster’s 1828 dictionary generally meant . . . “1. A profound secret; something wholly unknown or something kept cautiously concealed, and therefore exciting curiosity or wonder . . .”
Therefore, it not necessary that a mystery explained in the NT was not evident in the OT
Strong's # 3446
musthrion = from a derivative of muo (to shut the mouth); a secret or mystery.
Parables: Matthew 13:11, prophesied in Psalms 78.
Stars and Candlesticks explained: Revelation 1:20.
of God: Revelation 10:7, fulfilled in Revelation 11:15.
Babylon: Revelation 17:5-7.
musthria 1 Cor 13:2, 1 Cor 14:2
musthrion Rom 11:25, 1 Cor 15:51, Eph 1:9, Eph 3:3, Eph 5:32, Eph 6:19, Col 1:26, Col 4:3, 2 Th 2:7, 1 Ti 3:9, 1 Ti 3:16
musthriou Rom 16:25, Eph 3:9, Col 1:27, Col 2:2
musthriwn 1 Cor 4:1
musthriw 1 Cor 2:7, Eph 3:4
But is this a reach too far? Does the OT text speak of a blessing involving both Abraham and the Gentiles? First, a helpful insight.
The Greek term translated as gentiles in Ephesians and Galatians (ἔθνη) can also be translated as "nations."
Gentile is a Latin word, revealing the translators themselves. Anyone who's ever read an interlinear knows ethnos means a race, people, or nation. Ethnology is the study of races.
Is there then, some hint in the OT text of a blessing to Gentiles coming as part of God’s promises to Abrahaham. There a hint of this mystery – a mystery later to be fully revealed/explained in the NT.
Genesis 18:17-19
17 And the LORD said, Shall I hide from Abraham that thing which I do; 18 Seeing that Abraham shall surely become a great and mighty nation, and all the nations of the earth shall be blessed in him? 19 For I know him, that he will command his children and his household after him, and they shall keep the way of the LORD, to do justice and judgment; that the LORD may bring upon Abraham that which he hath spoken of him. KJV
Please tell me that you're aware of these verses:
John 8:38-40 [38] I speak that which I have seen with My Father: and ye do that which ye have seen with your father.[39]They answered and said unto Him, Abraham is our father. Ιησους saith unto them,
If ye were Abraham's children, ye would do the works of Abraham.[40]
But now ye seek to kill Me, a man that hath told you the truth, which I have heard of God: this did not Abraham. (seek to kill Him: Matthew 21:37-45, Matthew 23:34-37)
THIS is NOT figurative language:
Genesis 17:1-21 And Abram was ninety-nine years old, and the Lord appeared to Abram and said to him, I am thy God: be well-pleasing before Me and be blameless,[2] and I will establish My Covenant between Me and thee, and I will multiply thee exceedingly.[3] And Abram fell upon his face, and God spoke to him, saying,[4] And I, behold! My Covenant is with thee, and thou shalt be a father of a multitude of nations.[5] And thy name shall no more be called Abram, but thy name shall be Abraam, for I have made thee a father of many nations.[6]
And I will increase thee very exceedingly, and I will make nations of thee and kings shall come out of thee.[7] And I will establish My Covenant between thee and thy seed after thee to their generations for an Everlasting Covenant, to be thy God and the God of thy seed after thee. ...[21] I will establish My Covenant with Isaac...LXX
If any of this was about the gentiles, why not take a bride of the Canaanites?
Genesis 28:1-4 And Isaac having called for Jacob, blessed him and charged him, saying, Thou shalt not take a wife of the daughters of the Chananites.[2] Rise and depart quickly into Mesopotamia, to the house of Bathuel the father of thy mother, and take to thyself thence a wife of the daughters of Laban thy mother’s brother.[3] And may my God bless thee, and increase thee and multiply thee, and
thou shalt become gatherings of nations.[4] And may He give thee the blessing of my father Abraam, even to thee and to thy seed after thee, to inherit the land of thy sojourning, which God gave to Abraam.LXX
Genesis 28:10-14 And Jacob went forth from the well of the oath, and departed into Charrhan,[11] and came to a certain place and slept there for the sun had gone down; and he took one of the stones of the place and put it at his head and lay down to sleep in that place[12] and dreamed, and behold a ladder fixed on the earth whose top reached to heaven, and the angels of God ascended and descended on it.[13] And the Lord stood upon it, and said, I am the God of thy father Abraam and the God of Isaac: fear not, the land on which thou liest, to thee will I give it and to thy seed.[14]
And thy seed shall be as the sand of the earth: and it shall spread abroad to the sea and the south and the north and to the east, and in thee and in thy seed shall all the tribes of the earth be blessed.LXX
Therefore, these are the GENETIC seed of Jacob, coming home:
Isaiah 43:1-8 And now thus saith the Lord God that made thee O
Jacob, and formed thee O Israel, Fear not, for I have redeemed thee: I have called thee by thy name, thou art Mine.[2] And if thou pass through water, I am with thee; and the rivers shall not overflow thee: and if thou go through fire, thou shalt not be burned; the flame shall not burn thee.[3] For I am the Lord thy God, the Holy One of Israel, that saves thee: I have made Egypt and Ethiopia thy ransom, and given Soene for thee.[4] Since thou becamest precious in My sight, thou hast become glorious, and I have loved thee: and I will give men for thee, and princes for thy life.[5] Fear not, for I am with thee: I will bring thy seed from the east, and will gather thee from the west.[6] I will say to the north, Bring; and to the south, Keep not back:
bring My sons from the land afar off, and My daughters from the ends of the earth,[7] even all who are called by My name. For I have prepared him for My glory and I have formed him and have made him:[8] and I have brought forth the blind people, for their eyes are alike blind and they that have ears are deaf.LXX
"Gentile" is never substituted for "Jacob" ...so it's obvious who is meant... in context.
The circumcision Commandment
for Abraham's tribes to enter the Promised Land
is NOT repealed in the Gospel of the Kingdom of Heaven:
John 7:22-23 Moses therefore gave unto you circumcision; (not because it is of Moses, but of the fathers) and ye on the Sabbath day circumcise a man.[23] If a man on the Sabbath day receive circumcision that the law of Moses should not be broken, are ye angry at Me because I have made a man every whit whole on the Sabbath day?
Genesis 17:1-14 ...be well-pleasing before Me and be blameless... [9] And God said to Abraam, Thou also shalt fully keep My Covenant, thou and thy seed after thee for their generations.[10]
And this is the Covenant which thou shalt fully keep between Me and you, and between thy seed after thee for their generations; every male of you shall be circumcised.[11] And ye shall be circumcised in the flesh of your foreskin, and it shall be for a sign of a covenant between me and you.[12] And the child of eight days old shall be circumcised by you, every male throughout your generations, and the servant born in the house and he that is bought with money, of every son of a stranger, who is not of thy seed.[13] He that is born in thy house, and he that is bought with money shall be surely circumcised, and My Covenant shall be on your flesh for an Everlasting Covenant.[14] And the uncircumcised male, who shall not be circumcised in the flesh of his foreskin on the eighth day, that soul shall be utterly destroyed from its family, for he has broken My Covenant.LXX
If the gentiles don't want it, that's up to them. But proselytes coming into the land keep the laws of the land. They don't change the laws of the land to make the whole land gentile. Does Paul's gospel raise the gentiles or lower Jacob?