Ion-gated transistors have enabled us to electrically control electronic phase transitions such as superconductivity in a variety of materials. On the other hand, the carrier doping mechanism, particularly in oxide semiconductors, still remains elusive. Here, we report on the low-temperature thermopower of two-dimensional electron systems in ion-gated SrTiO3. We found that the metallic states induced by ionic gating exhibit a large peak in thermoelectric power to over 1 mV/K at around 20 K. This enhancement of cryogenic thermoelectric power is ascribed to a phonon-drag effect, which is in general dramatically suppressed by conventional chemical doping because the phonon mean free path is suppressed by chemical disorder. The large sustained peak, even in the high carrier density regime of 2x10(14) cm(-2), strongly indicates that a less disordered process, i.e., electrostatic charge accumulation, dominates over electrochemical carrier doping in the ionic gating in SrTiO3.