Carte got them together and wanted then to be partner and told them that they could be as big if not BIGGER than Offenbach! So Sullivan DID kinds "steal" ( but really it's all parody ), and so did Gilbert re bits and pieces of staging.
A LOT of Iolanthe was patterned on Wagner...but it was parody, so it wasn’t really stealing.
Also, the Mikado has hints of “Madame Butterfly”, and “I am a Courtier Grave and Serious” from the Gondoliers lifts verbatum a finger exercise from the Hanon Piano book.
I love doing parodies.