Genesis 12:1 Now the Lord had said to Abram: “Get out of your country, From your family And from your father’s house, To a land that I will show you. 2 I will make you a great nation; I will bless you And make your name great; And you shall be a blessing. 3 I will bless those who bless you, And I will curse him who curses you; And in you all the families of the earth shall be blessed.”
The word "you" in Genesis 12:3 applies to Abraham, not the modern nation of Israel. God promised to bless those who blessed Abraham and curse those who cursed him. Galatians 3:7–9 further explains that the promise, "In you all the nations shall be blessed," is fulfilled in those who have faith like Abraham, including the Gentiles.
Galatians 3:7 Therefore know that only those who are of faith are sons of Abraham. 8 And the Scripture, foreseeing that God would justify the Gentiles by faith, preached the gospel to Abraham beforehand, saying, “In you all the nations shall be blessed.” 9 So then those who are of faith are blessed with believing Abraham.