In Galatians 2, did St. Paul rebuke St. Peter, or was it someone else?

That was St. Peter. The only ones who would try to claim it was another person are liberal theologians who do not believe Paul wrote Galatians, or who do not believe the people named in the epistles are the actual people, thinking they are made up by someone posing as St. Peter because they have an agenda. This is exactly the kind of thing I cover in our video series, The Demise of Catholic Scripture Scholarship.