I had GD with my last two pregnancies (the first of which ended at 33 weeks, before I'd had a chance to get used to having GD!). With the last pregnancy I was testing myself at home once a week and the GD kicked in at 17 weeks.
The main tricks that helped me keep my numbers good were these:
1) Walking a good distance every day. Exercise helps type II diabetics, and GD is type II-like. I tried to get in 30 minutes at a decent pace every day (didn't always work, but I tried!)--two 15-minute walks spaced over the day is probably even better. And truly, little things like using stairs instead of an elevator (if you still can as your pregnancy progresses!
) are also helpful.
2) Eating small portions 6 or 7 times a day instead of regular portions at 3 meals.
3) Always combining protein and/or fat with carbs: a banana with peanut butter, cheese and crackers, meat and salad. I love fruits and vegetables, but the only way I could eat them was to make sure I had a glass of whole milk, some nuts, or some yogurt with them. (ETA: eggs may become your new best friend! They're also supposed to be great for developing brains. And this is also your chance to have a little bacon every day if you love it!)
Note also that you *will* be able to eat small quantities of ice cream and other sweets high in fat and protein. In fact, Ben and Jerry's ice cream was kinder to my blood sugar than a "healthy" bagel with lox and cream cheese! No sorbet or other carbs-only treats, though, and keep it to a small portion at a time.
4) Eat some cheese and crackers before going to sleep to keep your blood fasting levels low. Sounds crazy, but it works. (A PP already explained why, I think--if your body goes "too long" without gastronomic input, it releases sugars into your bloodstream to keep your blood sugar level from tanking, which in someone with GD can't be processed quickly enough to be back to fasting levels by morning.)
Remember also that your insulin resistance (your GD) will change throughout the pregnancy; it hits a maximum at about 32 weeks, I believe. So don't expect to figure out exactly what and when you can eat and use that for the rest of the pregnancy. I tested fasting and my 1 and 2-hour levels after every meal one day a week every week of my last pregnancy. If they were good, I knew I was still eating okay for my current insulin resistance. If they were a little high, I modified my eating (smaller portions, more protein, fewer carbs, . . . ) and kept testing until my levels were good.
Hope this helps, and good luck to you!