If you are at risk for GD, it does not 'automatically' mean that your baby will be a big baby BUT you are at risk for having a larger baby. Here's why:
During pregnancy, the extra hormones your body is making actually hinders the body's ability to use insulin. The pancreas sometimes cannot keep up with the demand, leading to GD. Improper levels of insulin mean that all the glucose won't be processed and the main concern is that it'll end up in the baby's blood. In turn, the baby's pancreas has to work harder to try and either process the glucose or make more insulin. Strangely enough this can cause the baby's body to create more fat, especially in the upper body area - a condition known as macrosmia (9 pounds, 15 ounces or larger as a general rule of thumb).
While it isn't a guarantee that you will have a large baby it is probably something you want to avoid (large babies can lead to c-section since the head might be too big, tearing/episiotomy, longer delivery/pushing, and trauma to the baby/broken bones during delivery). Start NOW! You're doing the right thing by inquiring about diabetic diets. Would your ob/gyn write a prescription for a nutritionist? Your insurance might pay for it.
In any case, try to eat 4-5 smaller meals throughout the day. You want to keep the sugar level in your body 'constant' as opposed to eating 3 big meals where the pancreas has to work overtime. Try to avoid processed foods, refined sugar, and junk food. Protein and certain types of dietary fibers are usually a good bet. Go to your Borders or B&N and search for diabetic cookbooks to get ideas. Good luck to you - I went on a diabetic-style diet in my 5th month of pregnancy and have never looked back, still adhering to the better eating habits I adopted them.
And...my baby arrived at 7 pounds 1 ounce. Furthermore, I didn't have to do anything special regarding my weight. I adhered to the same healthy eating habits I had adopted and shed my pregnancy weight within a mere 3-4 weeks.