Complex sentence: Gilbert had already written a considerable body of stories, plays, poems, criticism and other works before writing "Creatures of Impulse" and would go on to write the libretti to the famous Savoy operas (composed by Arthur Sullivan) between 1871 and 1896.
Split into simpler sentences: Gilbert had already written a considerable body of stories, plays, poems, criticism and other works before writing "Creatures of Impulse". He would go on to write the libretti to the famous series of Savoy operas (composed by Arthur Sullivan) between 1871 and 1896.