“Indeed when Gentiles, who do not have the law, do by nature things required by the law, they are a law for themselves”.

Read my translation again!

On Galatians 5:3, I would say Paul’s meaning is more down to earth: “If you think you need to be circumcised, then in order to be consistent with yourself, you need to keep the whole law, because circumcision is just an arbitrary selection.”

Otherwise stated, he is simply pointing out the illogicality of insisting on circumcision.

But in any case, the ones who were insisting on it were Jewish Christians, not gentile ones. They wanted circumcision implemented in all new Christian converts who were not Jewish as a sign that they had to become Jewish in order to receive grace and salvation. Paul’s argument against them is nothing to do with the Spirit but that circumcision for them was merely a symbolic act because they had no intention of keeping the whole law. The ones who were insisting on it were already Jewish and otherwise subject to the whole law anyway. So it was not wrong for Paul to suggest that they were under obligation to keep the whole of it. His argument in a nutshell: “You were born Jewish so you were subject to the law by nature but now Christ has set you free from the law and you are no longer subject to it. But if you want to go back on that and insist on circumcision, then you have to go back on the whole law, you can’t just go back on a small or merely symbolic part of it because law doesn’t work like that. Law is not something that you pick and choose about. It is something that you are under obligation to for you natural life (Rom 7:1).”

There is also a distinction between Jewish converts and proselytes. The proselytes did not become Jewish, they merely followed the tenets and joined in the communities of the Jews. The converts actually became Jewish (though some Jews denied the validity of that). The Judaisers who Paul was up against insisted on conversion to Judaism as a condition precedent to Christian baptism.