There are lines in scripture, and one of those lines is pre-Christ and post-Christ, but that has nothing to do with the Old Covenant and the New Covenant (neither of which have anything directly to do with Gentiles in any age). There was no Old Covenant for ~2000 years and it was given only to Israel. The New Covenant has not come in yet, and it too is given only to Israel.
The Mystery of Christ was unfolded from Genesis and then through the ages. All of these things made know "since" or "from" the foundation (overthrow) of the world (Rom 16). Paul then revealed the final Mystery which was hidden from "before" the foundation of the world (Eph).
Now to him that is of power to establish you according to my gospel, and the preaching of Jesus Christ, according to the revelation of the mystery, which was kept secret since the world began. (Rom 16)
According as he hath chosen us in him before the foundation of the world, that we should be holy and without blame before him in love. (Eph 1)
And to make all men see what is the fellowship of the mystery, which from the beginning of the world hath been hid in God, who created all things by Jesus Christ... (Eph 3)
Which in other ages was not made known unto the sons of men, as it is now revealed unto his holy apostles and prophets by the Spirit. That the Gentiles should be fellowheirs, and of the same body [sýssōmos], and partakers of his promise in Christ by the gospel... (Eph 3)
Whereof I am made a minister, according to the dispensation of God which is given to me [Paul] for you, to fulfill the word of God ; Even the mystery which hath been hid from ages and from generations, but now is made manifest to his saints.(Col 1:25-26)
This is not the same truth Paul was teaching in Galatians (Acts Age). It is not that Gentiles would be blessed through Abraham. That is truth revealed in scripture and known since it was promised to Abraham.
And the scripture, foreseeing that God would justify the heathen through faith, preached before the gospel unto Abraham, saying, In thee shall all nations be blessed. (Gal 3:7)
Surely, it was not understood how or when or why, but it was plain to see. It came to pass when Gentiles were "grafted" into Israel (Romans 11), but that grafting no longer occurs in this age. All believers are essentially Gentiles.
For this cause therefore have I called for you, to see you, and to speak with you: because that for the hope of Israel I am bound with this chain.
(Acts 28 to the Jews at Rome)
For this cause I Paul, the prisoner of Jesus Christ for you Gentiles, If ye have heard of the dispensation of the grace of God which is given me to you-ward.
(Ephesians 3, Post-Acts)
We must draw straight lines (2 Tim 2:15), and the OT/NT split confuses that calling. As I understand it, it was a creation of Jerome for the Vulgate and has carried into all translations.
We need to be careful with things which are not inspired. Take them as opinions (at best) and examine all by the whole counsel of God.