The gentiles did not first receive the word of GOD in the New Testament, so this happening in the book of Acts was not some radical idea that never existed in Judaism before. (Jesus came to fulfil the Law; Matthew 5:17) GOD gave Israel the oracles of GOD, but GOD wants those voices heard by all people. This is even an Old Testament teaching through the prophet Jonah.
Ironically, probably because they weren't Jews, Jonah did not want to do what GOD asked. When he did do what GOD asked, it was very successful and the city repented and turned to GOD. Nineveh was actually a city of Arab people to the best of my knowledge. With the other prophets of GOD in the Old Testament the formula is usually the reverse; The prophet is very willing, but the people are slow to listen.