Well, I should have been more specific. Yes, Abraham's promise did include many nations. However, throughout the entire OT it was the Jews who were God's chosen people. As Galatians says, the Gentiles were without God and without hope.
That's Ephesians 2:11-12, not Galatians.
In other words, they were definitely not part of God's chosen people, those destined to inherit the land promised to Abraham, Isaac, and Jacob.
Yes, they were and they still are today! Paul says Gentiles believers are "fellowheirs" of God's promises. Why are you trying to make it so that they are not fellowheirs? That mystery was there, but hidden in the OT and revealed in the NT. It isn't that it wasn't taught in the OT, it was just not taught clearly there on purpose because it was God's plan for it to be revealed after sending His Son to the earth. But, that mystery hidden in the OT scriptures was revealed in the NT by the NT authors whose spiritual eyes were opened by the Holy Spirit to understand the OT prophecies.
Here is one example of an OT prophecy that refers to the Gentiles also being God's chosen people which no one realized until Paul clarified it.
Isaiah 11:9 They shall not hurt nor destroy in all my holy mountain: for the earth shall be full of the knowledge of the Lord, as the waters cover the sea. 10 And
in that day there shall be a root of Jesse, which shall stand for an ensign of the people; to it shall the Gentiles seek: and his rest shall be glorious.
I wouldn't be surprised if you, as a premil, think this passage is about a future earthly kingdom established when Christ returns. But, it's not.
Romans 15:8 Now I say that
Jesus Christ was a minister of the circumcision for the truth of God, to confirm the promises made unto the fathers: 9 And that the Gentiles might glorify God for his mercy;
as it is written, For this cause I will confess to thee among the Gentiles, and sing unto thy name. 10 And again he saith, Rejoice, ye Gentiles, with his people. 11 And again, Praise the Lord, all ye Gentiles; and laud him, all ye people. 12
And again, Esaias saith, There shall be a root of Jesse, and he that shall rise to reign over the Gentiles; in him shall the Gentiles trust.
Can you see here how Paul quoted Isaiah 11:10 and applied it to the Gentiles being saved by what Jesus Christ did for them? No one understood what passages like those were talking about until it was revealed in the NT. And no one understood that Gentile believers would be fellowheirs with Israelite believers of God's promises made to Abraham.
Galatians 3:16 Now
to Abraham and his seed were the promises made. He saith not, And to seeds, as of many;
but as of one, And to thy seed, which is Christ...29 And if ye be Christ's, then are ye Abraham's seed, and heirs according to the promise.
Why do you talk in terms of Israel inheriting promises God made to them as if He made promises to Israel that don't also apply to the Gentiles? Why is all this still a mystery to you even though it is revealed in the NT?
But fast forward to the NT, and God reveals something He had up His sleeve all along, namely that the Gentile would in fact be fellowheirs and of the same body with the Jews. Notice that being fellowheirs and of the same body does not mean the church replaced Israel. They instead became one and the same. Nobody replaced anybody.
Gentiles being fellowheirs means that whatever God promised to Israel to inherit is promised to Gentile believers, also. But, you don't seem to believe that. You talk in terms of God fulfilling promises only to Israel for some reason despite the fact that Gentile believers are fellowheirs of those promises.