GD happens regardless of your diet. I had been eating healthy, was still working (and I delivered pizzas, so I was constantly on my feet and walking around) cut out soda, the whole nine yards. The placenta is what causes you to get GD. You can have it one pregnancy and not the next or vise versa. I didn't have it with my first daughter, but I do with this one. The doctor will better explain it to you, but it was nothing you personally did or didn't do to cause it to happen. ALSO, it isn't technically caused because of sweets. (Which I love!) when you get put on a diet for your GD, they will try to control it with a diet at first, and it will be a very strict low-carb diet. The carbs is what gets your blood sugar levels elevated. Carbs turn into sugar after they are digested, and if you have GD, excessive amounts of carbs can cause extra sugar to not get digested by your pancreas, and it just floats around in your blood stream. Not too lovely. I wouldn't worry too much though. They will inform you on everything you need to know about it, especially if you have a lot of questions for them to answer. And they will do everything possible to help you control it!
I was underweight with my first pregnancy and also didn't eat a lot of sweets, and I ate healthy and still failed the one hour and 3 hour glucose tests. Not horribly, but enough to send me to the nutritionist. i also was exercising and active.
Up until pregnancy I'd had bouts of hypoglycemia, where my sugar would plummet if I hadn't eaten in a while, and honestly, having higher blood sugar was a welcome relief.
I followed the diet meticulously and delivered a very healthy 7 pound 3 oz boy.
It's just one of those things that seems to be common in pregnancy - higher blood sugar levels in some women.
Sounds like you'll be fine with a careful diet, as the above poster says. Interestingly, fruit juices were a no-no. Too much sugar at once.