Hmm, I'm going to respond at the risk of offending those very AP...
GD is not something to be taken lightly. I think it would be copyright infringement to cut and paste text from OB textbooks (which I have access to online), so I'll paraphrase. The following is referenced from Gabbe
bstetrics-normal and problem pregnancies. 2002
First: why try to identify GD mammas?
Mainly to decrease the risk of morbidity and mortality in the baby. The high sugar, and yes, high ketones levels alluded to above, are very clearly associated with birth defects and baby loss/miscarriage (also other things like free oxygen radical excess). Some of the birth defects are very serious. Fetal macrosomia, or a big baby, is not considered a birth defect but can complicate delivery.
Certain studies show that of mammas diagnosed with GD, about 50% will develop diabetes mellitus down the line, anywhere from 5-20 y depending on whose study you read and how long they followed the patients.
But a huge, huge part of diabetes is diet and lifestyle. Yes, certain people may be predisposed to getting it (because of family genes) but current thought is that it doesn't develop without the right setting (diet).
Pregnancy is a state of increased insulin resistance, which means just because your fasting glucose is okay, doesn't mean that your body can handle a glucose load well. Hence the need for the 2 hour, and if necessary the 3 hour tolerance test.