I am looking for RI of gold that is coated on a transparent substrate. The gold has thickness of 40-60 μm. Can I simply select the RI (refer to refractive index database) from the operated wavelength?
Yes. This gold film is thick enough that the optical properties approach the bulk form. Be aware that the real part of the RI is < 1: for example ~ 0.2 @ 632.8 mm.