My sister (who is also diabetic) is awaiting gall bladder removal. Her's was infected so she spent a week in hospital while on IV antibiotics. They stopped her Metformin (2 x 500mg) and put her on a fat free diet. Her HbA1c dropped to 5.1 and she has remained there despite not taking the medication.
Her diet was very much Mediterranean but currently she is on a seriously bland diet until the gall bladder is removed. Basically no fats, no dairy, no citrus fruits, low sugar and only water to drink - so she is reduced to poached chicken or white fish with steamed veggies. She can normally tolerate some cereals but dry cereal is not a lot of fun. She is reduced to a bit dry toast for breakfast as eggs are off the menu for now too.
They did say once the gall bladder is out she can return to her previous diet. She is getting pretty demented though with the diet and she is hoping that she will be called next week to get it out as that will be 6 weeks (they were waiting for the inflammation to come down so they can do keyhole surgery).