The time-averaged behavior of gap flow between two stationary side-by-side circular cylinders immersed in the subcritical Reynolds number regime and its variation with gap spacing are presented. A series of experiments and numerical simulations are performed. Results reveal that gap flow, which is the flow passing between the cylinders, can be classified broadly into pressure- and momentum-driven regimes, depending on the spacing ratio (T/D), where T is the transverse center-to-center spacing between the cylinders and D is the cylinder diameter. The pressure-driven regime occurs for roughly T/D<1.25, where the mean velocity of the gap flow increases as the spacing ratio increases. The momentum-driven regime follows with a monotonic decrease in the mean velocity as the spacing ratio increases when T/D>1.25. Within the pressure-driven regime, subtransitions of the gap flow are further classified, based on distinct changes in the circumferential static pressure distribution, as well as the velocity profile, transverse pressure gradient, and mean velocity at the throat of the two cylinders.