“Christ, in the council and covenant of grace and peace, asked many things of his Father, which were granted; he asked for the persons of all the elect to be his bride and spouse, and his heart's desire was given him, and the request of his lips was not withheld from him: he asked for all the blessings of grace for them; for spiritual life here, and eternal life hereafter; and all were given him, and put into his hands for them, Ps 20:2; and here it is promised him, and I shall give thee the Heathen for thine inheritance, and the uttermost parts of the earth for thy possession; by "the Heathen", and "the uttermost parts of the earth", are meant God's elect among the Gentiles, and who live in the distant parts of the world; which are Christ's other sheep, the Father has given to him as his portion, and whom he has made his care and charge: as if it was not enough that he should be King of Zion, or have the government over his chosen ones among the Jews, he commits into his hands the Gentiles also; see Isa 49:6; and these are given him as his inheritance and possession, as his portion, to be enjoyed by him; and who esteems them as such, and reckons them a goodly heritage, and a peculiar treasure, his jewels, and the apple of his eye. These words respect the calling of the Gentiles under the Gospel dispensation; and the amplitude of Christ's kingdom in all the earth, which shall be from sea to sea, and from the rivers to the ends of the earth.”
― John Gill, Gill's Bible Commentary