I was also diagnosed with GDM during my first pregnancy. However I was not prescribed with insulin jab. Instead my doctor asked me to control my diet and check my blood glucose using glucometer daily before and after meals. Diet was simple: wholemeal bread with cheese or margarine and milk/zero sugar soya milk for breakfast, brown rice with veggie and meat for both lunch and dinner. For fruits wise, only those not so sweet like green apples, oranges etc. Need to avoid watermelon and mangoes etc as this will cause spike in blood glucose level.
As long as you don't eat fried food, food that made of flour and starch, sugary drink, dessert, very sweet fruits such as mangoes, grapes, watermelon etc then your reading should be below 6.8.