From Wikimedia; filled in shadows enough to show sleeves..
She became a widow in 1626 and this appears to be widow’s dress.
Keywords: Caterina di Ferdinando de' Medici, Caterina de’ Medici, Catherine de' Medici, duchessa di Mantova e di Monferrato, governatore di Siena, Medici family, Gonzaga family, Duchess, Italian, curly coiffure, ribbon headdress, earrings, high enclosing neckline, deep wide ruff, bodice, tabbed hanging sleeves, long false sleeves, lace-edged back-flared cuffs