To know the accurate relationship between pore size and the pressure of capillary condensation or evaporation and also to elucidate the pore-connectivity effects on adsorption hysteresis and pore criticality for the ordered mesoporous silicas (MCM-48 and KIT-6) with bicontinuous gyroid structure, we carried out X-ray structural study of MCM-48 and KIT-6 and measurements of the adsorption-desorption isotherms of nitrogen in comparison with MCM-41 and SBA-15 with unconnected cylindrical pores. The results clearly show that the hydraulic diameter describes incorrectly the mean pore size of these materials expressing the strength of confinement under the assumption of cylindrical pores with no networking effects. In addition, the average pore diameter at the middle point of pore intersections still underestimates the strength of confinement compared to the unconnected cylindrical pores of MCM-41 and SBA-15. Despite a large difference in porous structure, the shape and thermal behavior of the adsorption hysteresis for KIT-6, as well as the sorption scanning behavior, were indistinguishable from those for SBA-15, indicating that interconnections among pores of almost the same size do not have a significant effect on the adsorption hysteresis and pore criticality.