At the CAOM in Aix-en-Provence BMD are available until 1900 for the city of Gros-Morne (Martinique).
When was your grandfather born approximatively ? If he was born after this datenothing can be done at the CAOM and you'll have to write to the city of Gros-Morne and ask for a copy of his birth certificate or marriage certificate. You only have to write to the :
Mairie de Gros-Morne, Service de l'Etat-Civil 972 Martinique (FWI) Supply an envelope for the answer and the price of the copy and stamp and you'll have your certificate