to be specific, untreated gas turbine exhaust contains a lot of nitrogen oxides. similar thing (bar particulates) also happens with diesel engines, but usually there’s a catalytic converter downstream. venting it like this is also a massive waste of energy because another ~half of power is still recoverable in steam turbine, for example in my city there’s large CCGT plant with ~350MW gas turbine, ~150MW steam turbine coupled to it and another ~400MW waste heat is pushed into municipal heating grid. smaller turbines are much less efficient than that