As to your question- many Gentiles actually joined with Israel, and God explains how they were to be accepted, and what the requirements are for them should they want to stay.
My daily reading is taking me through the Book of Joshua now, which recounts God fighting for Israel to remove those who live in the land He promised Abraham to give to his descendants.
Previously, we read that many of the groups/nations that lived there had developed a real fear of Israel, because word had spread throughout the land as to what God had done in Egypt, and at the Red Sea. The event at the Red Sea was too monumental to not have had many witnesses.
However, though they had learned that Israel intended to assume possession of their lands, only one group, the Gibeonites, acted accordingly, though deceptively. Using deception, they worked out a treaty of sorts with Israel. That deception resulted in them becoming servants to Israel, but they did not end up being defeated in war.
So, all of these nations/small city-states knew what was going on, they knew of the eyewitnesses of the parting of the Red Sea, AS WELL AS the stopping of the Jordan River (Joshua 3), as well as previous defeats that took place before Israel crossed over this river. Instead of seeking "Israel's" God (He is God of all), they instead chose to band together to go to war with them.
After the first group was completely defeated, another group of kings banded together, only to face the same consequence. God explains why these peoples were being ousted from where they lived- they had long ago rejected Him, and were completely immersed in idolatry and immorality. Seeing their doom headed their way showed just how far they had sunk- they continued to defy the Living God after seeing over and over that God was fighting for Israel.
Everyone gets a chance/chances to know that God exists (Romans 1 & Acts 17:26-31), and to choose to do the right thing (repent and believe). Many, like you, do not like the terms that God requires, in spite of the fact that He has paid the ultimate price needed. Nor do you like how God metes out justice and mercy, as if you are smarter than God Himself
(cf. https://tinyurl.com/y4m9c528 ).
You look at those, like myself, who were radically changed, and you poo-poo this, because you believe you are smarter than those who "fall for this fantasy", but then, if you can poo-poo God Almighty, Christians are no big deal.