Have you been able to set up a consultation with a dietician? Your doctor should be monitoring these levels and should be directing you to insulin if needed. Insulin is usually the best option for women who have gestation diabetes and need a little help, although they usually try to get you to track what you are eating and your levels for a short time first- to see if you can control it with your diet.
What types of foods are you eating? Are you staying away from carbs? At night, before bed, do you have any snacks? If not- work on getting high protein foods into your diet, and cut back on the carbs. Your fasting sugars can usually be brought down by having a good protein/low carb snack a little bit before bed.
Reducing carbs may help - replace with protein and healthy fats.
Yes, if yourvsugars continue to be above target, insulin will be your best choice for normalising blood sugars.