What is 'Gn'S'?
'G n'S', or 'Gilbert and Sullivan' refers to the Victorian theatrical partnership of the librettist W.S.Gilbert (1836–1911) and the composer Arthur Sullivan (1842–1900) and to the works they jointly created. The two men collaborated on fourteen comic operas between 1871 and 1896. 
In Chronological order, they are...
Thespis; or, The Gods Grown Old: 1871.
Trial By Jury: 1875.
The Sorcerer: 1877.
H.M.S. Pinafore; 1878.
The Pirates of Penzance; 1879;
Patience; 1881.
Iolanthe; 1882.
Princess Ida; 1884.
The Mikado; 1885.
Ruddigore; 1887.
The Yeomen of the Guard; 1888.
The Gondoliers; 1889.
Utopia, Limited; 1893.
The Grand Duke; 1896.
Arthur Sullivan (left) and W.S Gilbert.