A gate assembly has a frame with a sill and an opening for the passage of a fluid, and a gate slide mounted on said frame for movement toward and away from said sill, between open and closed positions. The improvement, in combination therewith, comprises a first flange projecting from the frame on both sides of the opening, a resilient sealing member fixed to the first flange, and a second flange substantially identical to the first flange projecting from the back side of the gate slide. The first and second flanges are so positioned that, when the gate slide is in the closed position against the sill, the first and second flanges overlap and the resilient sealing member is compressed so as to produce a water tight seal. Sealing is accomplished by a compression of the resilient sealing member between the first and second flanges when the gate slide is substantially closed, thus virtually eliminating frictional wear of the resilient sealing member the first and second flanges are inclined, resulting in engagement and compression of the resilient sealing member as the gate slide approaches its closed position.