I suspect you are inadvertently calling interp2, rather than interpn. interp2 does require that the data be as if produced by meshgrid. In any case, you should show the code you have run so that we can reproduce it.
Ultimately, though, there is no reason to use either ndgrid or meshgrid when working with interpn or griddedInterpolant. It consumes extra memory unnecessarily and is, I believe, purely for backward compatibility. You can just call interpn with the original 1D grid vectors used as arguments to ndgrid. This is true in the CPU version and should be true on the GPU as well.
