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Gd 3 Ga 5 O 12 single crystals [gadolinium gallium garnet (GGG)] were shock compressed to elastic deformation, elastoplastic transformation, and structural phase-transition regimes with the corresponding single, two-wave, and three-wave structures. Velocity profiles at the front interface and the rear free surface of the sample are measured by a Doppler pin system. Results of these measurements are analyzed in detail, and the refractive index, n, at 1550 nm is obtained as a function of shock pressure or density. Incorporating previously published single-wave data above 100 GPa, characteristic variations in the refractive index are presented and discussed within the context of shock-induced polymorphism of the GGG.