Gregor directed several German-language theaters, including in Barmen-Elberfeld from 1898 to 1905. In Berlin, he led the Komische Oper as its director from 1905 to 1911. Gregor was noted as initiating the trend in opera productions where the stage director began to assert greater influence compared to the previous dominance of opera singers. In addition, the company provided a more naturalistic style of opera productions. He presided over several notable productions, including 19 premieres.
^Cowden, Robert H., "Acting and Directing in the Lyric Theater: An Annotated Checklist" (March 1974). Notes (2nd Ser.), 30 (3): pp. 449-459.
^Shaffer, Peter, "Book Guide Die Komische Oper, Berlin, 1947-54" (Winter, 1955-1956). Tempo (New Ser.), 38: pp. 29-32.
^Black, Leo, "Up Schmidt Creek" (June 1993). The Musical Times, 134 (1804): pp. 329-331.
^Giger, Andreas, "Tradition in Post World-War-I Vienna: The Role of the Vienna State Opera from 1919-1924" (December 1997). International Review of the Aesthetics and Sociology of Music, 28 (2): pp. 189-211.
^Klein, Herman, "Book Reviews: Die Welt der Oper - Die Oper der Welt (October 1932). Music & Letters, 13 (4): pp. 439-440.