Diabetes is caused (in theory), by too much glucose (a type of sugar) in the blood, and in most cases gestational diabetes develops in the second or third trimester then disappears after the baby is born. However, women who develop gestational diabetes are more likely to develop type 2 diabetes later in life so its something you should try to avoid, I have seen women take the view that if its just during pregnancy then why worry!!
However, the measurement of blood sugar is flawed as it only takes into account one kind of sugar (glucose), via the glucose tolerance test (GTT). At best it is only about 50% accurate, and it doesn't take into consideration the ratio between your total carbohydrates and your glucose.
Insulin is the preferred method of treatment that the medical profession use and I've written before on the topic. I urge you to ask for for proof that insulin will benefit you as I've yet to meet anyone classified as "diabetic" that actually got better taking insulin. In the clie The tolerance of glucose and your total carbohydrate has little to do with pumping you full of extra insulin.Check my previous posts on Diabetes to understand how it really works.