This study explores the effect of Gouy phase shift correction on determining the thickness and refractive index of concentric spherical shells mimicking the cornea measured by quasioptical terahertz (THz) spectroscopy. Reflections are measured from a focused Gausian beam in the 220-330 GHz spectrum with frequency-independent confocal distance for equal radius of curvature. Thickness and refractive index are estimated from the measurements using Fresnel's equations and a fixed phase velocity. The parameters are estimated a second time where the frequency and axial location dependent phase velocity is corrected by incorporating the expected Gouy phase shift. The Gouy phase correction is proven to bring thickness and refractive index values closer to reference in all cases investigated.