Did the Old Testament prophets know that Gentiles would be saved and how?
The Old Testament prophets had an inkling that God would do something and, especially from Isaiah onward, understood that salvation would extend to the Gentiles. However, they did not know how God would accomplish it, and there is no indication Abraham foresaw the Messiah's death, burial, and resurrection.
Les teaches from Galatians 3 that Scripture, foreseeing that God would justify the heathen through faith, preached the Gospel beforehand to Abraham, saying, “In thee shall all nations be blessed.” He stresses that God's foreknowledge sees the end from the beginning. Les rejects the idea that Abraham knew the death, burial, and resurrection of Christ, saying there is nothing in Scripture indicating Abraham foresaw the Messiah's death on a cross, burial, and resurrection. Instead, the Old Testament prophets had only an inkling that God was going to do something. They understood, especially from Isaiah onward, that God would have salvation for the Gentiles, but they did not know how. He then turns to 1 Peter 1:10 for further study.