Sad to hear that the GM opened with only one restaurant - must be really boring. As it's now off-season, even the good/popular restaurants will often look deserted and not appealing. From my experience, here are two good restaurants in Grand Baie:

Fusion
Fancy upscale restaurant offering fusion food. Coming from the south, follow the main Grand Baie coastal road heading north, on the right side close to the TOTAL petrol station there's a basic Italian restaurant called "Luigi" on the ground floor (the pizza is good though), the Fusion is on the first floor.

Don Camillo
Upscale Italian restaurant (= no pizza ).
Main Coastal road on the right side, at the CALTEX petrol station.

There are good restaurants in Mont-Choisy, too, which is closer to the GM. I have no recommendations as I stay in GB and I'm usually too lazy to drive there.