From Benford's Gilbert and Sullivan Lexicon:
The period between The Yeomen of the Guard and its eventual successor, The Gondoliers, was an extremely trying one for the triumvirate. The Queen’s perhaps offhand remark about grand opera was taken to heart by Sullivan and Carte. Gilbert wisely declined their invitation to join in such a venture, but Carte was convinced that English opera would appeal to English audiences. He therefore planned and started building a large opera house and urged Sullivan to write a serious opera that would assure the venture’s success. While this was going on, Gilbert started work on a new comic opera. Sullivan was in Venice, and he, Gilbert, and Carte were burdening the mails with a series of increasingly troublesome letters. Sullivan clearly felt himself imposed upon and complained to Carte that “excepting during the vocal rehearsals and the two orchestral rehearsals I am a cipher in the theatre.” Carte was brave (or foolish) enough to show the letter to Gilbert, who thereupon stopped work on the new libretto and wrote a blistering yet flattering letter to Sullivan, which perhaps helped clear the air. Eventually Carte brought the two back together and Sullivan found himself composing serious opera and comic opera simultaneously. The new comic opera was The Gondoliers; it opened at the Savoy on December 7, 1889, and ran for 554 performances.
The Gondoliers reflects the reconciliation between the two artists. Sensitive to Sullivan’s desire for more emphasis on music, Gilbert provided a cornucopia of opportunities for musical invention, and Sullivan responded with some of the most joyous melodies in all of musical theater. Further reflection on their relationship is seen in the two kings who reign jointly “as one individual.” The Gondoliers will surely long remain a solid favorite with music lovers everywhere.