The reference is to the Palatine Gallery in the Pitti Palace in Florence. The Grand Duke Ferdinand II employed Pietro Cortona to remodel the Palatine Gallery; Cortona's Baroque painting is set in elaborate gold and white stucco frames. This Gallery houses paintings collected by the Grand Dukes.
In a letter of 8 June 1845 there is an account of Ruskin's disappointment at the pictures in the Pitti Palace, particularly those of Salvator Rosa.