The achievement of H2 detection, up to 25 ppm, at room temperature using sulfur-treated, platinum (Pt)-decorated porous GaN is reported in this study. This achievement is attributed to the large lateral pore size, Pt catalyst, and surface treatment using organic sulfide. The performance of H2-gas sensors is studied as a function of the operating temperature by providing an adsorption activation energy of 22 meV at 30 ppm H2, confirming the higher sensitivity of the sulfide-treated Pt-porous GaN sensor. Furthermore, the sensing response of the sulfide-treated Pt-porous GaN gas sensor increases with the increase in porosity (surface-to-volume ratio) and pore radii. Using the Knudsen diffusion-surface reaction equation, the H2 gas concentration profile is simulated and fitted within the porous GaN layer, revealing that H2 diffusion is limited by small pore radii because of its low diffusion rate. The simulated gas sensor responses to H2 versus the pore diameter show the same trend as observed for the experimental data. The sulfide-treated Pt-porous GaN sensor achieves ultrasensitive H2 detection at room temperature for 125 nm pore radii.