Your answer makes no sense. The Church is just being formed. Jesus has not even been crucified yet. He is instructing Peter and the Apostles on the Church He has just stated is to be built upon Peter, the Rock. His statement, treat the sinner, who has rejected the Church, like a Gentile is a reference to Jewish practice towards Gentiles at the time, which is to say that one who rejects the Church is to be treated as unclean, an unrepentant sinner. It is not a directive on how to treat Gentiles. It is a directive on how to treat those who reject the authority of the Church.
Makes you wonder why anyone would want to reject His Church and remain outside of it.
You're wrong, of course...Jesus told the apostles NOT to preach to Gentiles...There wasn't a Gentile in the bunch when Jesus made the reference to Gentiles...Hence, no Catholic church...