I had GD last time, failed my one hour and miraculously passed my three hour this time. Everyone is right about what to avoid: simple starches, carb heavy stuff. My diet was really strict, 30 g of carbs for meals and 15 g for snacks with a 3 meal, 2 snack schedule. The total carbs per day is not as important as the spacing, and not binging. Binging is the worst, because then the baby staarts to produce insulin to process the sugars your body can't. Insulin is a growth hormone, and that is why the baby gets big. My midwife finally explained it all to me and I thought it was pretty interesting.
But here is my real advice. EXCERCISE. Even a short walk around the block after a meal is a good thing. I swam like 3-4 times a week when I had GD and it had such an effect on my blood-sugar levels. It's just about the best thing.
Also to keep your stress in check. To me this was killer becuase the whole GD thing so freaked me out, I think it actually made things worse.
I did have a big girl, 9.1 lbs, 2 weeks late. But she was perfectly healthy, and the birth was ideal...no tearing, no drugs, all good. Dood luck to you. You will be very healthy when you have this baby!