Complex sentence: There is controversy among some art historians about one of the paintings by Metsu, now in the Gemäldegalerie, which was already known in the 19th century as "Familie des Kaufmanns Gelfing".
Split into simpler sentences: There is controversy among some art historians about one of the paintings by Metsu, now in the Gemäldegalerie. In the 19th century it was already known as "Familie des Kaufmanns Gelfing".