Capillarity in small, confined pores has a pronounced effect on the depression of the dissociation temperature of gas hydrates, known as the Gibbs-Thomson effect. However, this effect remains poorly understood in natural fine-grained sediments with wide pore size distributions. This study investigated the effect of pore size distributions of fine-grained sediments on the dissociation temperature of a gas hydrate. A gas hydrate was synthesized under partially water-saturated conditions in nanosized silica gels and in various natural fine-grained sediment samples, including sand, silt, diatoms, a diatom-sand mixture, and clayey sediment. The synthesized hydrate samples were thermally dissociated under isochoric conditions, while the melting temperature depression and the shifted phase boundaries were monitored. We observed a dissociation temperature depression of approximately 0.1-0.3 degrees C in silt, 0.2-0.4 degrees C in the diatom sample, and 1.2-1.5 degrees C in clayey silt, while no temperature depression was observed in sand. In a particular case of diatom-sand mixture, the dual porosity condition with the submicron-scale internal pores of diatoms and the macropores of sands rendered dual phase boundaries, one with an similar to 0.4 degrees C temperature depression and one with no depression, respectively. Despite the wide ranges of pore size, gas hydrates were preferentially formed in smaller pores, which comprise less than 40% of the cumulative pore volumes. This was because the initial water loci exacerbated the Gibbs-Thomson effect in partially water-saturated conditions. Our results provide clear experimental evidence on and novel insights into the effect of pore size distributions of fine-grained sediments on the dissociation behavior and phase boundaries of gas hydrates, both in the presence of free gas and in water-limiting conditions that exhibit a considerable Gibbs-Thomson effect.