The title compound, K-GaSi-GIS, potassium gallium silicon oxide hydrate, was synthesized hydro­thermally and its crystal structure was determined from data collected on a single crystal of dimensions 10 × 10 × 8 µm at a synchrotron X-ray source. The compound, which has the aluminosilicate (AlSi) zeolite gismondine (GIS) topology, Ca4[Al8Si8O32]·16H2O, crystallizes in the tetragonal space group I41/a. A disordered distribution of the framework Si/Ga sites leads to higher symmetry of the GIS-type network compared with the usual monoclinic symmetry in AlSi-GIS. Framework Ga substitution for Al in AlSi-GIS leads to substantial distortion of the crankshaft chains, reducing the effective pore dimensions and suggesting the possibility of pore-dimension control via partial framework-cation substitution.