The title of the person in the kitchen who is responsible for creating sauces and gravies of all kinds is sauté chef
The sauté chef is also known as the saucier and is responsible for preparing all sauces and gravies that accompany other dishes in the kitchen.
The other options are not correct:
Grillardin: The grillardin is responsible for grilling meats, vegetables, and seafood.
Boucher: The boucher is responsible for butchering meats.
Poissonnier: The poissonnier is responsible for preparing fish and seafood.
I hope this helps!