I'm wondering if there are any passages/prophecies about salvation for the Gentile world in the Old Testament. If you know any, please share.
YES, there are!
The footnote in my NLT Bible for Acts 11:1 has a list of Old Testament verses that say that salvation is for all people, not just the Jews. Below, I've reproduced the list and for two of them, I've directly quoted them because they're the clearest and most direct with regards to this topic.
Isaiah 49:6 (NLT)
6 He says, “You will do more than restore the people of Israel to me.
I will make you a light to the Gentiles,
and you will bring my salvation to the ends of the earth.”
11 Many nations will join themselves to the Lord on that day, and they, too, will be my people. I will live among you, and you will know that the Lord of Heaven’s Armies sent me to you.
The first promise to the Gentiles was given to Adam and Eve (Gen.3:15) who were not Jews. The second promise was given to Japheth, the son of Noah who was father to the Gentiles (Gen.9:27).Shem was the father of the Jews (Gen.9:26). This was later confirmed by Isaiah (Isa.54). And I might add two more things. The word Jew was given to Israel by the Gentiles after the Tribe of Judah. The Jewish people are actually of Syrian decent according to Scripture (Due.26:5). As for praying for Israel we should pray as we do for all men but asking God to bless Israel is a sin. Note (2 John 2:7-11). Only those that are in Christ are of Israel "seed" (Gal.3:16) and (Gal.3:27-29). Let Gods Word be true and every man a liar that does not agree with it. God bless those that are in Christ the Son of God.
Only the true children of Israel, in whom were of a dark skin or so called negroes in whom ONLY fit ALL the curses of Deut 28, will receive salvation. Until you understand the history of ISRAEL then you will think that salvation is for non israelite gentiles. To avoid a lot of scriptures here are three that should cover everything. John 3:5-6 lets us know if you are not born of the Spirit you cannot see the Kingdom. Joel 2:28 states that he will pour out his spirit on all flesh referring to the Israel. If you don't believe the all flesh is Israel only here is an extra one to prove that. ACTS 2:39 is to Israel only and to all of Israel that was not there. Just do some homework and you will see that all the scriptures given above are not precepts just random scripts with no true understanding. Now the last script is Jer 31:31-37 in which the Most High made a covenant only with Israel and Judah not spiritual Israelites because there is no such thing. Now if you don't believe he is talking about Israelite strangers, Israelite gentiles, here is a bonus. Amos 9:9 lets us know that Israel is in every nation but he will keep track of each on of us. Shalom