A gate valve assembly for shearing an elongated member and controlling a fluid flow through a through bore of the gate valve assembly, wherein the through bore is adapted for fluid communication with a well bore and is configured for receiving the elongated member, wherein the gate valve assembly comprising: at least one gate element (2, 5) movably arranged in a direction transverse to the through bore, wherein the gate element (2, 5) is provided with a cutting portion for shearing the elongated member in the through bore: first and second seal seats (12, 13) arranged for sealing the at least one gate element (2, 5); a seal assembly provided with at least one sealing surface (10, 11) for sealing with at least one of the first and second seal seats (12, 13), wherein the seal assembly comprises sealing activation means for providing additional sealing force in the engagement between the at least one sealing surface (10, 11) of the seal assembly and at least one of the first and second seal seats (12, 13); characterized in that the seal assembly is arranged in a portion of the gate element (2, 5) which is located outside the through bore in an open position of the gate element (2, 5) and is displaceable in the transverse direction into a sealing position when the gate element (2, 5) is in a closed position, and wherein in an activation mode of the seal assembly the sealing activation means provides the additional sealing force in the engagement between the at least one sealing surface (10, 11) of the seal assembly and at least one of the first and second seal seats (12, 13). at least one gate element (2, 5) movably arranged in a direction transverse to the through bore, wherein the gate element (2, 5) is provided with a cutting portion for shearing the elongated member in the through bore: first and second seal seats (12, 13) arranged for sealing the at least one gate element (2, 5); a seal assembly provided with at least one sealing surface (10, 11) for sealing with at least one of the first and second seal seats (12, 13), wherein the seal assembly comprises sealing activation means for providing additional sealing force in the engagement between the at least one sealing surface (10, 11) of the seal assembly and at least one of the first and second seal seats (12, 13); characterized in that the seal assembly is arranged in a portion of the gate element (2, 5) which is located outside the through bore in an open position of the gate element (2, 5) and is displaceable in the transverse direction into a sealing position when the gate element (2, 5) is in a closed position, and wherein in an activation mode of the seal assembly the sealing activation means provides the additional sealing force in the engagement between the at least one sealing surface (10, 11) of the seal assembly and at least one of the first and second seal seats (12, 13). first and second seal seats (12, 13) arranged for sealing the at least one gate element (2, 5); a seal assembly provided with at least one sealing surface (10, 11) for sealing with at least one of the first and second seal seats (12, 13), wherein the seal assembly comprises sealing activation means for providing additional sealing force in the engagement between the at least one sealing surface (10, 11) of the seal assembly and at least one of the first and second seal seats (12, 13); characterized in that the seal assembly is arranged in a portion of the gate element (2, 5) which is located outside the through bore in an open position of the gate element (2, 5) and is displaceable in the transverse direction into a sealing position when the gate element (2, 5) is in a closed position, and wherein in an activation mode of the seal assembly the sealing activation means provides the additional sealing force in the engagement between the at least one sealing surface (10, 11) of the seal assembly and at least one of the first and second seal seats (12, 13).