What DNA test can i do to get clues about my maternal grand father?

Your own DNA will give you no clues at all without a comparison sample from the said grandfather.

One of your maternal grandfather, or if he's not available, one of your mother and your maternal grandmother that would exclude those markers in your mother's DNA that came from your maternal grandmother. Simply getting a DNA test of yourself would be rather useless since only about a quarter of your DNA would come from him and there'd be no way to tell what without getting DNA tests from all of your other grandparents and doing a comparative analysis.