Hba1c is not an accurate measure in pregnancy, that too at 23-28 weeks. Once your GTT is abnormal, it is gestational diabetes. You can control it with diet and exercise alone or treatment depending on sugar values. Also you can ideally buy a glucometer and check sugars at home - fasting and post meal(1 hour) daily. Fasting should be less than 90 and post meal (1 hr) should be less than 140.