A loading/unloading method for a gas turbine system is disclosed. The gas turbine system includes a combustion section featuring a primary combustion stage with a first plurality of fuel nozzles and a downstream, secondary combustion stage with a second plurality of fuel nozzles. For loading, the method progresses through each of a plurality of progressive combustion modes that sequentially activate a higher number of at least one of the first or second plurality of fuel nozzles; and for unloading, the method progresses through each of a plurality of progressive combustion modes that sequentially activate a lower number of at least one of the first or second plurality of fuel nozzles. During each combustion mode, regardless of whether loading or unloading, a primary combustion stage exit temperature of a combustion gas flow is controlled to be within a predefined target range corresponding to the respective combustion mode.