Like his brother Leopoldo, Gian Carlo was a cardinal. However, in comparison with his brothers, he was a man of far less disciplined instincts, though not without taste. He invited Salvator Rosa, whom he had met in Rome, to come to Florence where he was paid an annual income to paint for the Court while remaining free to accept commissions from other patrons. Gian Carlo also provided funds for a company of actors to build a theatre in the Via della Pergola, and for another company he rented a palazzo in the Via del Cocomero for which Ferdinando Tacca was asked to design sets and scenery. 
Gian Carlo's true interests were food, which he consumed in immense quantities, and women, whom he pursued with the insatiable lust of a satyr. Expelled from Rome for refusing to be accompanied by older and less libidinous cardinals on his visits to Queen Christina of Sweden, he returned to Florence still young, rich and good-looking, exquisitely dressed, his hair long and curly, determined to devote himself to pleasure. He moved into a beautiful villa built in the middle of an entrancing and exotic garden off the Via della Scala. 
Here he made love to a succession of mistresses, often, it was said, to several at once, and had at least one tiresome rival drowned in a carp pond. 
He once ordered the release of a notorious murderer, whose wife he had immediately taken to bed when she had come to him on her husband's behalf, and threatened to cut off the Sheriff's head if his order was not obeyed. The Sheriff appealed to the Grand Duke who stood in silence for a few moments before resignedly declaring, "Obey the Cardinal, since he is my brother." Everyone knew that Ferdinando II was frightened of Gian Carlo and, when the news was brought that he had died of apoplexy, Ferdinando received it with evident relief rather than sorrow. 

"The Rise and Fall of The House of Medici", Christopher Hibbert.