Don't you have access to the OT? You do seem to be a specialist in the law and prophets.
Indeed I do, and that's not what it says. If you make a claim about what the Bible says, then you should be able to quote it.
What does Paul say in Romans chap 2-8? Doesn't that agree with Moses? Please don't pretend you 've no idea what I'm talking about.
Romans 2:6-8 He will render to each one according to his works: 7 to those who by patience in well-doing seek for glory and honor and immortality, he will give eternal life; 8 but for those who are self-seeking and do not obey the truth, but obey unrighteousness, there will be wrath and fury.
These verses are fully in agreement with Moses and in favor of those doing good works in obedience to the Mosaic Law and are against those who do not obey it. In Romans 2:13, only the doers of the Law will be justified. In Romans 2:26, the way to tell that a Gentile has a circumcised heart is by observing their obedience to the Law, which is the same way to tell for a Jew (Deuteronomy 10:12-16, 30:6)
What law exactly are gentiles under? Is it the law given only to Israel? If so, how? You must show how non Israeli are subject to a law not given to them. Was murder sin before that law? Was adultery sin before that law? The law given to Israel didn't invent sin. Paul says the body of flesh isn't subject to the law.
God's Law refers to the Law that God has given, which everyone is under because God is sovereign, and I listed some examples where Gentiles were under God's Law and were obligated to obey it.
You are so focused on whom the Law was given to that you've lost sight of whom it was given by. There are many verses that describe the Mosaic Law as being instructions for how to walk in God's ways, such as Deuteronomy 10:12-13, Isaiah 2:2-3, Joshua 22:5, Psalm 103:7, and many others, so the Law was not given as instructions for how to live as a Jew, but as instructions for how to express God's character traits, such as holiness, righteousness, goodness, justice, mercy, faithfulness, love, joy, peace, patience, kindness, gentleness, and self-control. Jesus expressed these character traits through his actions and what that looked like was complete obedience to the Mosaic Law, so that is what it should look like when he is living in us. Our sanctification is about being made to be more like Christ, to have and to express the same character traits, so there is no sense in Gentiles wanting to be made like him while wanting nothing to do with what he was like.
So while the Law was given to Israel, it was never intended only for Israel, but rather it was given to Israel in part so that they would be equipped to be a light to the nations and bless them through teaching them to repent from theirs sins and how to walk in God's ways in obedience to His Law (Isaiah 2:2-3, 49:6). In Deuteronomy 4:5-8, the intended reaction from the nations of seeing Israel's obedience to God's Law was to marvel and how great and wise God is. In other words, the Law was intention to be used as a tool to evangelize the nations and draw them into a covenant relationship with God. There is no sense in a Gentile wanting to become a follower of God while refusing to follow the instructions that He gave for how to follow Him.
Indeed, there are many examples of God's laws being given throughout Genesis prior to when they were given. Sin was in the world before the Law was given (Romans 5:13), so there was was nothing that became sinful when the Mosaic Covenant was made or that ceased being sinful after it has become obsolete, but rather the Law revealed what has always been and will always be sinful. So if under the New Covenant you believe that we should still refrain from what God has revealed to be sin, then you should believe that we should still obey His Law.