An expanding gate valve and method is provided for bidirectional sealing capability against pressure, whether the direction of pressure is upstream or downstream. The bidirectional annular seat seals against the gate assembly against pressure from the normal wellhead direction, typically upstream. For sealing against pressure in the opposite direction, the seat is formed with an annular recess extending radially outwardly from the central bore of the annular seat so that a flex portion of the seat formed between the recess and the front sealing face of the annular seat at the central bore flexes toward the gate assembly against this pressure. The front face of the bidirectional seat forms a raised annular sealing portion on the flex portion of the seat adjacent the central bore to provide sealing contact with the gate assembly in the expanded condition when the flex portion flexes toward the gate assembly.