Isaiah prophesies and confirms the gentiles are included in the promise!
Isaiah 56:3-5
3 Do not let the son of the foreigner (anyone outside the ancient or natural nation of Israel, the gentile)
Who has joined himself to the Lord
Speak, saying,
“The Lord has utterly separated me from His people”; (Do not let the gentile say this to himself)
Nor let the eunuch say,
“Here I am, a dry tree.” (Do not let the gentile say this to himself)
4 For thus says the Lord:
“To the eunuchs who keep My Sabbaths,
And choose what pleases Me,
And hold fast My covenant, (anyone who does these things God will accept, Jew or gentile alike).
5Even to them I will give in My house
And within My walls a place and a name
Better than that of sons and daughters; (who could this be other then natural descendants of Israel)
I will give them an everlasting name
That shall not be cut off.
When Israel left the captivity of Egypt, many Egyptians left with them and became assimilated into the nation!
Exodus 12;
37 Then the children of Israel journeyed from Rameses to Succoth, about six hundred thousand men on foot, besides children. 38 A mixed multitude went up with them also, and flocks and herds—a great deal of livestock.
Joshua spared Rahab the harlot and her family, non-Israelite, but they assimilated and excepted into the nation. Joshua 6
In the book of Esther, during the Medo-Persian empire, many non Jews joined the Jews in their celebrations regarding the Jew vindication from their enemies;
Esther 9:27
the Jews established and imposed it upon themselves and their descendants and all who would join them, that without fail they should celebrate these two days every year, according to the written instructions and according to the prescribed time,
And don't forget about Ruth the Moabitess! Don't forget the Levitical laws that concern the foreigner that they are to be treated the same, and after the 10th generation can participate in temple services unto the Lord.
Paul clearly teaches that the coming and work of Christ has broken down the barriers between Jew and gentile.