“How and where do we start applying the Mosaic law (and the conditions of it) to gentiles?”
We can never apply the Mosaic Law to gentiles in the original purpose of the Law as the conditional covenant between God and people. This contract was only for the nation of Israel.
Yet, the Law is more than just a contract. The content of the Law also defines the Holiness of God. (Leviticus 19:2, Exodus 33:20). As such, it showed Israel how morally different God was from them. This principle of God being different from people applies to more than just Israel. Since Israel was made up of human beings, we can extrapolate this to the entire human race. God is different on a moral level than the entire human race. And, in fact, for Israel and for us, it shows our moral ineptness and sinfulness (Isaiah 59:1, Jeremiah 17:9). This is why Paul, in Romans 3:20 can say to Jews and Gentiles (in the Roman Church), “For by works of the law no human being will be justified in His sight, since through the law comes the knowledge of sin.”
This is also why Jesus says in Matthew 5:17-18, “Do not think that I have come to abolish the Law or the Prophets, I have not come to abolish them, but to fulfill them. For truly, I say to you, until heaven and earth pass away, not an iota, not a dot, will pass from the Law until all is accomplished.”
And again, Paul explaining the Law to the church, says in Galatians 3:24, “Therefore the Law has become our tutor to lead us to Christ, so that we may be justified by faith.”
The reality is this: the Law still has a purpose and as such will be in effect until all is accomplished. That purpose is to show humans their sin nature and need for Jesus.