Process for the operation of a gas turbine group (1) producing energy by combustion of a gaseous fuel (24) with a mixture of air gas. There is interconnected with the group (1) an air gas separation unit (7) producing at least two air gases (9; 10-13). The operating conditions of the turbine (3) of the group are maintained similar to those of maximum power production by selecting, with the aid of the separation unit (7), the parameters of the mixture of air gas. The combined assembly for the production of energy and at least one air gas comprises a gas turbine group (1), comprising a first compressor (2) coupled to a turbine (3) and a combustion circuit (5) between the outlet of the first compressor (2) and the inlet of the turbine (3); an air separation unit (7) comprising a second air compressor (14) connected to an air supply inlet (8) of the unit, an impure nitrogen outlet (9) and at least one other outlet (10-13) for an air gas; and structure (25, 26; 20, 23) selectively to connect the air inlet (8) to the outlet of the first compressor (2) or the impure nitrogen outlet (9) to the combustion circuit (5).