Hydraulic fracturing is essential for efficient developments of low-permeability reservoirs. For these fractured production wells, serious water channeling frequently occurs because of the fracture connections to injectors or underlying/lateral aquifers. To address the problem, the preformed gel treatment is typically adopted. One of the key mechanisms for the treatment is gel dehydration, which has however not been well understood. This study presents a lab experimental study of the mechanisms of gel dehydration and its impact on water shutoff performance. We carried out both sand pack and fractured core tests to comprehensively study how capillary pressure and viscous pressure induce gel dehydration. Results show that viscous-pressure-induced dehydration greatly enhances water shutoff performance through forming a concentrated gel cake with a width less than 3 mm, whereas capillary force acts oppositely and reduces the bonding between gel and fracture surfaces. Specifically, viscous pressure dominates the early stage of gel dehydration and capillary force takes over in the later stage while the influence of viscous pressure fades away due to the formation of the gel cake. Increasing the viscous pressure can effectively offset the damage caused by capillary pressure, which favors gel dehydration and thereby improves water shutoff performance. This work, to our best knowledge, is the first that particularly provides insight into the synergistic mechanism of capillary force and viscous force dehydrating a polymer gel. The overall work enhances our understanding of the interplay of viscous and capillary pressures on gel dehydration and its fracture shutoff performance. It provides a theoretical basis for designing gel injection strategies during field treatment in fractured reservoirs.