Maybe your thinking with regard to Gentiles surrounds the passage wherein Jesus told his Disciples not to bring the good news to the Samaritans or Gentiles. Matthew 10:5
Jesus sent out these twelve, instructing them as follows:[m] “Do not go on a road that leads to Gentile regions[n] and do not enter any Samaritan town.[o] 6 Go[p] instead to the lost sheep[q] of the house of Israel. 7 As you go, preach this message: ‘The kingdom of heaven is near!’
Sure enough, our Lord
said that.
But why?
The answers are always in scripture TF. And all scripture is always about Jesus/God.
Zechariah14: 9
The Lord will then be king over all the earth. In that day the Lord will be seen as one with a single name.[o] (Notice? It doesn't say, king over Israel only).
Remember, the old testament
is not done away with.
The old testament is the foundation of prophecy of Messiah and the new testament is fulfillment of that prophecy with Messiah come.
Why did Jesus say that? Because it was old testament prophecy of Messiah's mission once born to fulfill it. That's the message to Abram. Pay attention to the bolded last part of Genesis 12 excerpt below. This is why you are mistaken to think Gentiles are not included in the plan of salvation or are those other sheep in the sheepfold.
Genesis 12:1
Now the Lord said[a] to Abram,[b]“Go out[c] from your country, your relatives, and your father’s household
to the land that I will show you.[d]2 Then I will make you[e] into a great nation, and I will bless you,[f]
and I will make your name great,[g]so that you will exemplify divine blessing.[h]3 I will bless those who bless you,[i]but the one who treats you lightly[j] I must curse,so that all the families of the earth may receive blessing[k] through you.”
Any and all blessings come through the nation of Israel.
The promise in Genesis (above) fulfilled. The OT tells us in different books there that God always intended for the Gentiles to receive his free irrevocable gift.
Isaiah 61,
Jeremiah 3:15-18. Just two examples for you.
Luke 2:30 For my eyes have seen your salvation[v]
31 that you have prepared in the presence of all peoples:[w]
32 a light,[x]for revelation to the Gentiles,and for glory[y] to your people Israel.”
Romans 15:7 Receive one another, then, just as Christ also received you, to God’s glory. 8 For I tell you that Christ has become a servant of the circumcised[
d] on behalf of God’s truth to confirm the promises made to the fathers,[
e] 9 and thus the Gentiles glorify God for his mercy.[
f] As it is written, “
Because of this I will confess you among the Gentiles, and I will sing praises to your name.”[
g] 10 And again it says: “
Rejoice, O Gentiles, with his people.”[
h] 11 And again, “
Praise the Lord all you Gentiles, and let all the peoples praise him.”[
i] 12 And again Isaiah says, “
The root of Jesse will come, and the one who rises to rule over the Gentiles, in him will the Gentiles hope.”[
j] 13 Now may the God of hope fill you with all joy and peace as you believe in him,[
k] so that you may abound in hope by the power of the Holy Spirit.