The relative permittivity of gold is 6.9. Relative permittivity is the ratio of the dielectric permittivity of a material to the permittivity of a vacuum, and therefore has no units.
The relative permittivity of a material is also known as the material's dielectric constant. If the material is used as the dielectric material in a capacitor, then the material's relative permittivity is the ratio of the capacitance of such a capacitor to the capacitance of an otherwise identical capacitor that used a vacuum as its dielectric material. Relative permittivity is often represented by the Greek letter epsilon or an uppercase or lowercase letter "k."