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We investigate the impacts of random dopant (RD) and gate workfunction variability on the subthreshold characteristics of a 50-nm-gate-length inversion-mode gate-all-around In0.53Ga0.47As MOSFET using a 3-D finite-element quantum-corrected drift-diffusion device simulator calibrated to experimental data. We have studied threshold voltage, off-current, and subthreshold slope variations. The workfunction variations on the subthreshold characteristics dominate and decrease with the reduction in grain diameter. The simulated grain diameters of 10, 7, and 5 nm exhibit threshold voltage standard deviations of 52, 41, and 27 mV, respectively. These values are larger than those observed in TiN-metal-gate Si FinFETs for a similar gate length. The impact of RD fluctuations is negligible when compared with bulk Si MOSFETs, giving a threshold voltage spread of only 6 mV.