If a structure is flexible (NF < 1 Hz), then we must calculate the flexible gust factor.  If the structure has a stiffness which is significantly different in each direction,  do we still base the gust factor on the lowest natural frequency for the entire structure?  Or is the intent to determine the lowest frequency in the direction that is under consideration, and use that Frequency for the gust factor calculation?  

For example, if a structure has a Frequency of 0.7 Hz when looking at a direction, and 0.35 hz in the perpendicular direction, do we still use 0.35 Hz for the basis of G used in for wind pressures in both directions?   

Determine G independently for each direction based on the predominant mode in that direction.