A gas flow diverter (10) receives large volumes of hot gases from a gas turbine (11) and has a blade (26) adapted to close either the outlet port (10A) to a heat recovery steam generator (12) or the outlet port (10B) to a stack (13). Each port is surrounded by an open channel the two side walls of which are each provided with a seal (24) with both exposed, one beyond the other. The surfaces (40) of the blade are stainless steel cladding and are connected through insulation (41) to a reinforcement (41A) in a manner permitting but confining the expansion of the cladding. The frame (32) has a border providing first and second ledges (32A and 32B) on each side, which are spaced and positioned so that each may engage a different one of the two seals (24) bordering an outlet port when closed, thus establishing the channel as a closed passageway surrounding the closed port which may be sealed against leakage by discharging sealing air into it. The blade (26) is shifted between its two operative positions by means of a reversible drive (46) including toggle joints (49) connected to the blade reinforcement (41A) by a unit (50) which is shiftable in response to forces exerted by the expanding or contracting toggle joints (49).