Are there Old Testament Scriptures that mention salvation for the Gentiles? 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.


Genesis 12:3
Psalm 22:27
Isaiah 42:4
Isaiah 49:6



  Isaiah 49:6 (NLT)
  6 &nbsp;&nbsp;&nbsp;&nbsp; He says, “You will do more than restore the people of Israel to me.
  &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; I will make you a light to the Gentiles,
  &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; and you will bring my salvation to the ends of the earth.”



Isaiah 56:3-7
Isaiah 60:1-3
Jeremiah 16:19-21
Zechariah 2:11



  Zechariah 2:11 (NLT)   
  
  11&nbsp;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.



Malachi 1:11
Romans 15:9-12 (These are different wordings of Psalm 18:49, Deuteronomy 32:43, Psalm 117:1, and Isaiah 11:10.)