Please always indicate the question as detailed as possible in the post title, I have changed your title.

Gaussian supports density fitting technique to accelerate calculation, however in Gaussian this technique only supports for pure functional. Since the wB97XD is a hybrid functional, it is completely meaningless to specify fitting set in the present case, so you should leave this option blank.

I couldn't understand what you want to mean by "when i choose this method gaussian the fitting set parameter". Please explain, and please also provide some background to your question.