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.