We report light-transmittable ultrasmooth gold film enabling gap-mode tip-enhanced Raman scattering (TERS) spectroscopy measurements with extremely low background signals. An ultrasmooth Au substrate was fabricated by introducing a wetting layer of germanium (Ge). We also found that high mechanical and chemical stability of the substrate along with flexibility in chemical modification provides versatile platforms for gap-mode TERS measurements in various environments, such as air, water, and organic solvents.