Paul’s letters were to the early Christian church which were largely NOT Gentiles at that time. In fact, there was a lot of disagreement as to whether Gentiles would be allowed to convert to Christianity. This is especially evident in Galatians.
I’ll grant that this is mostly outside of my scope of knowledge, but that sounded wrong and a quick look through the Wikipedia would lead me to believe otherwise:
He is principally writing people in Greece, and a few churches that were started early in his mission in Turkey. Outside of Hebrews, it doesn’t look to me like he is writing to any non-Gentiles in any of his letters, and certainly not in a majority of them.
If he was doing so, we would expect him to be writing to people in Israel, Syria, Lebanon, Jordan, and Egypt principally. If he’s barely even making his way into Turkey, I don’t think it’s reasonable to expect that he’s mostly talking to Jewish people.
It is interesting that you allude to the letter to the Galatians which, itself, refers to Paul’s battle to be allowed to preach to and convert Gentiles.
I have no idea where the idea that the audience that Paul addressed might have been Jewish Christians (or Christians who sprang from the earth with neither Jewish nor Gentile antecedents). Paul was writing in the 50s when only a very few members of his audience could have been young enough to have been “born Christian.” Everyone was raised either Jewish or Gentile (i.e., non-Jewish).
I have never seen anyone assert that Paul was not preaching and writing to anyone other than Gentiles. That is one of his most often used sobriquets: “Apostle to the Gentiles.”