Complex sentence: In 1936 Gauld returned to the United States to sing at the Met again through 1938 where his biggest success was as Méphistophélès in "Faust".
Split into simpler sentences: In 1936 Gauld returned to the United States. He returned tot the Met again in 1938 where he had his biggest success at the house as Méphistophélès in "Faust".