A gas turbine is operated using a varying blend of a first fuel, preferably natural gas, and a second fuel that is hydrogen. The hydrogen concentration is varied depending on operating conditions in order to reduce emissions of CO and NOx, and/or to mitigate LBO. The fuel mixture is varied using a controller based on a combination of factors in a modular operation concept to address different issues according to relevant load limitations. A method of operating a gas turbine according to this modular operational concept is also provided.