Neurite microenvironment has been approximated as impermeable, thin cylindrical tubes. This assumption has been validated in human white matter by a power law with the exponent of around 0.5 at b-values above 4000 s/mm2. However, the decay exponent in gray matter deviates from 0.5, suggesting that the cylindrical tube approximation does not apply in gray matter. This study aimed to study the whole-brain gray matter distribution of the decay exponent, and demonstrated an apparent contrast in the decay exponent between cortical gray matter and deep gray matter. This suggests that inherent microstructural differences may exist between these gray matter regions.