Souls of Gentiles came about after the sin of Adam:
If not for the sin of the Eitz Hadaas, the soul of Gentiles would not have been created, and all people would have a G-dly soul. Originally, Adam’s soul incorporated only Jewish souls, until he sinned. His sin caused him to now also include and bring about souls of gentiles.
 Likkutei Torah of Arizal, brought in Likkutei Sichos Chayeh Sara 15 footnote 17