Insulin (whether injected or created naturally) is required to control your glucose level. If your body isn't creating and processing it correctly (which is the case if you have GD) and diet hasn't worked, insulin is needed. As your pregnancy progresses, it is harder and harder to control your glucose readings.
My problem was my readings right after lunch and dinner but morning and after breakfast was fine. I had to take the long lasting insulin in the morning and it helped but towards the end it was less effective.
You could maybe try more protein in you evening snack and less of whatever else you are eating at that time. Or maybe making the snack a little earlier. Becareful though because if your diet change does work and you have taken the insulin as scheduled, your number could drop too low during the night.
How far along are you?