We report on the rapid optical detection of gaseous hydrogen using hot electrons generated from resonantly excited substrate-based gold nanohemispheres (Au NHs). We consider hot electron induced H2 dissociation and the subsequent formation of a metastable gold hydride (AuHx) to account for changes in optical transmission. The excitation wavelength was varied to demonstrate a maximum response at the localized surface plasmon resonance (LSPR) wavelength of the AuNHs. Numerical simulations, using the discrete dipole approximation, were employed to corroborate the optical changes associated with the formation of metastable AuHx. Finite time difference domain (FDTD) calculations were also performed to account for the enhanced photocatalytic activity arising due to the confinement of electric fields by the Au NHs. FDTD simulations show that the excitation of the Au NHs plasmon modes generates stronger electric fields at the interface in comparison to a spherical geometry of similar dimensions.