This paper proposes a method based on the gust loading factor (GLF) for evaluating the along-wind response of wind turbine system. The approach presented differs from conventional GLF methods as the GLF contains contributions from two resonant modes, mainly due to blade/tower interaction effects. The wind turbine system model considered contains two interconnected flexible subsystems, representing the tower and the rotor system. Each component is initially modeled as a separate degree-of-freedom (DOF), and these are coupled together to form a two DOF reduced order model of the coupled blade/ tower system. Thus, the resonant component of the response contains energy output from the two modes of the coupled system. The GLF is obtained for both tower tip displacement and base bending moment through numerical integration, with a closed form expression included for the former. A series of numerical examples are included to investigate the magnitude of GLFs obtained for the two DOF assembly which allows for blade/tower interaction, and these are compared with GLF values obtained from an equivalent single DOF model which ignores blade/tower interaction by lumping the mass of the blades in with that of the nacelle.