All of the areas of Galatia were parts of the Roman empire originally settled by the Celts.
According to Prof. Hans Volkmann's article on Galatia in Der Kleine Pauly (1975), under the Roman empire the Roman province (under a governor of praetorian rank) included a number of other districts besides that settled by the Celtic Galatai and the different meanings of "Galatia" make it hard to determine if the addressees of Paul's letter to the Galatians were in the entire Roman province or in the strictly Celtic Galatian area.