what were his first plays?
"Uncle Baby", "Hush-a-Bye, Baby, On the Tree Top, or, Harlequin Fortunia, King Frog of Frog Island, and the Magic Toys of Lowther Arcade" , "Dulcamara, or the Little Duck and the Great Quack", and others.
W. S. Gilbert
 "Uncle Baby", which ran for seven weeks in the autumn of 1863.
In 1865–66, Gilbert collaborated with Charles Millward on several pantomimes, including one called "Hush-a-Bye, Baby, On the Tree Top, or, Harlequin Fortunia, King Frog of Frog Island, and the Magic Toys of Lowther Arcade" (1866). Gilbert's first solo success came a few days after "Hush-a-Bye Baby" premiered. His friend and mentor, Tom Robertson, was asked to write a pantomime but did not think he could do it in the two weeks available, and so he recommended Gilbert instead. Written and rushed to the stage in 10 days, "Dulcamara, or the Little Duck and the Great Quack", 
what is h.m.s. pinafore? A comic opera in two acts. who created this? W.S.GIlbert where was he born? 17 Southampton Street, Strand , London.
wiki:16772589
W. S. Gilbert [SEP] First plays
Gilbert wrote and directed a number of plays at school, but his first professionally produced play was "Uncle Baby", which ran for seven weeks in the autumn of 1863. In 1865–66, Gilbert collaborated with Charles Millward on several pantomimes, including one called "Hush-a-Bye, Baby, On the Tree Top, or, Harlequin Fortunia, King Frog of Frog Island, and the Magic Toys of Lowther Arcade" (1866). Gilbert's first solo success came a few days after "Hush-a-Bye Baby" premiered. His friend and mentor, Tom Robertson, was asked to write a pantomime but did not think he could do it in the two weeks available, and so he recommended Gilbert instead.