The origins and meaning of the Grau Surname:
German and Jewish (Ashkenazic): nickname for someone with gray hair or a gray beard, from German grau ‘gray’.
Southern French: topographic name for someone who lived near a canal giving access to the sea, Occitan grau (Latin gradus ‘step’).
French: from Old French grau denoting a type of agricultural fork with curved tines (apparently of Germanic origin), hence a metonymic occupational name for a maker, seller, or user of such implements.
Catalan: topographic name from grau (Latin gradum).
Catalan: from a reduced form of the common medieval personal name Guerau (see the Gerald Surname).