Question: What opera that Gerd Grochowski performed in was the last of four music dramas?

Context articles: Whitney Houston videography
Das Rheingold
Ghost Opera
Richard Wagner
Siegfried (opera)
Maria Callas
Four Music
Götterdämmerung
Clarence Whitehill
Gerd Grochowski

The videography of American pop/R&B recording artist Whitney Houston consists of fifty-five music videos, four music video compilations, a concert tour video and three music video singles.
 In 1983, Houston signed a recording contract with Arista Records and two years after released her eponymous debut album.
 Houston's first music video was for the single "You Give Good Love", which was selected to establish her in the black marketplace first.
 In the video of worldwide hit "Saving All My Love for You", she played a beaming All-American girl shadowed by her secret lover's wife.
 The following video "How Will I Know", directed by Brian Grant, that helped introduce the singer to a wider audience when it became one of the first videos by a black female singer to earn heavy rotation on MTV, blasting open the doors for a whole generation of R&B and pop divas to follow.
 The clip won MTV Video Music Award for Best Female Video at its 3rd ceremony of 1986. "
Greatest Love of All", the final single released from Houston's debut album, which helped cement the M.O. for the classic Whitney video.
 In June 1986, Houston released her first video compilation "The No. 1 Video Hits", containing her four music videos off the "Whitney Houston" album.
 The video compilation reached number-one on the "Billboard" Top Music Videocassettes chart and stayed at the top spot for 22 weeks, which remains the all-time record for a video collection by a female artist, and was certified Platinum for shipments of 100,000 units by the Recording Industry Association of America (RIAA) on July 15, 1986.
 In 1987, "I Wanna Dance with Somebody (Who Loves Me)", the first single from her second album "Whitney", was one of Houston's most recognized music videos in company with the song's smash hit worldwide.
 Houston's fashion and hairstyle in the clip―towering curly wig, colorful dangly earrings and a series of going-to-the-club outfits―became one of her iconic looks.
Das Rheingold ( ; The Rhinegold), WWV 86A, is the first of the four music dramas that constitute Richard Wagner's "Der Ring des Nibelungen," or in English, 'The Ring of the Nibelung'.
Ghost Opera is the eighth studio album from metal band Kamelot.
 It was released in 2007 by SPV GmbH/Steamhammer Records, on June 1 in Germany and followed by releases on June 4 in Europe and June 5 in the United States.
 It is the first studio album by Kamelot to feature keyboardist Oliver Palotai, and the last with bassist Glenn Barry.
 The album spawned four music videos for the songs "Ghost Opera", "The Human Stain", "Rule the World" and "Love You to Death".
Wilhelm Richard Wagner ( ; ] ; 22 May 181313 February 1883) was a German composer, theatre director, polemicist, and conductor who is primarily known for his operas (or, as some of his later works were later known, "music dramas").
 Unlike most opera composers, Wagner wrote both the libretto and the music for each of his stage works.
 Initially establishing his reputation as a composer of works in the romantic vein of Weber and Meyerbeer, Wagner revolutionised opera through his concept of the "Gesamtkunstwerk" ("total work of art"), by which he sought to synthesise the poetic, visual, musical and dramatic arts, with music subsidiary to drama.
 He described this vision in a series of essays published between 1849 and 1852.
 Wagner realised these ideas most fully in the first half of the four-opera cycle "Der Ring des Nibelungen" ("The Ring of the Nibelung").
Siegfried, WWV 86C, is the third of the four music dramas that constitute "Der Ring des Nibelungen" ("The Ring of the Nibelung"), by Richard Wagner.
 It premiered at the Bayreuth Festspielhaus on 16 August 1876, as part of the first complete performance of "The Ring" cycle.
Maria Callas, Commendatore OMRI (Greek: Μαρία Κάλλας ; December 2, 1923 – September 16, 1977), was a Greek-American soprano, and one of the most renowned and influential opera singers of the 20th century.
 Many critics praised her "bel canto" technique, wide-ranging voice and dramatic interpretations.
 Her repertoire ranged from classical "opera seria" to the "bel canto" operas of Donizetti, Bellini and Rossini and further, to the works of Verdi and Puccini; and, in her early career, to the music dramas of Wagner.
 Her musical and dramatic talents led to her being hailed as "La Divina".
Four Music ("FOUR MUSIC Productions GmbH") is a German record label founded by Die Fantastischen Vier in 1996.
Götterdämmerung ( ; Twilight of the Gods), WWV 86D, is the last in Richard Wagner's cycle of four music dramas titled "Der Ring des Nibelungen " ("The Ring of the Nibelung", or "The Ring" for short).
 It received its premiere at the Bayreuth Festspielhaus on 17 August 1876, as part of the first complete performance of the "Ring".
Clarence Eugene Whitehill (November 5, 1871 - December 19, 1932) was a leading American bass-baritone.
 He sang at the Metropolitan Opera from 1915 to 1932.
 He sang on both sides of the Atlantic and is best remembered for his association with the music dramas of Richard Wagner, and for his recordings of well-known music hall ballads such as "I'll take you home again, Kathleen".
Gerd Grochowski (28 February 28, 1956, Krefeld – 16 January 2017, Mainz) was a German operatic bass-baritone who had an active international career from 1986 until his death in 2017.
 Particularly known for his performances in the operas of Richard Wagner, his roles included Donner in "Das Rheingold", Gunther in "Götterdämmerung", Klingsor in "Parsifal", Kurwenal in "Tristan und Isolde", Telramund in "Lohengrin", and Wotan in "The Ring Cycle".
 A graduate of the Hochschule für Musik und Tanz Köln, he was a longtime resident artist at the Cologne Opera.
 He appeared in leading roles as a guest artist at the Bayreuth Festival, the Berlin State Opera, the Bavarian State Opera, the Frankfurt Opera, the Hamburg State Opera, La Scala, the Liceu, the Linz State Theatre, the Stuttgart Opera, the Teatro Real, the Theater an der Wien, and the Salzburg Festival.