Gires–Tournois interferometer mirrors are designed and deposited. Because of deviation between the measured group-delay dispersion and the designed character, based on the measured layers' thickness, we fit group-delay dispersion measured values to analyse the errors produced during the deposition. When considering thickness deviation and the refractive index inhomogeneity, the fitted curve has a good agreement with the measurement result. Therefore, thickness errors and refractive index inhomogeneity are the main factors in manufacturing the Gires–Tournois interferometer mirrors. At the same time, the fitted actual manufactured thickness of every layer and the degree of refractive index inhomogeneity are obtained from the fitted curve.