There are websites devoted to this topic, and your doctor should be able to give you materials about diabetes and avoiding it during pregnancy. It is definitely true that women with diabetes who are super, super careful with their diet and all during pregnancy reduce the chance that their child will later get diabetes, so it is an effort worth making.
The quick answer is to eat "healthy" foods (high in fiber, lean means, fruits, veggies etc). Avoid junk food (fatty foods, highly processed foods, foods rich in sugar etc).
As AnnieBrooke mentioned, there are websites devoted to this topic. Try googling "gestational diabetes diet plan" or something along those lines.
I just met with the gest. diabetes clinic yesterday and got my meter and food chart and all that. When the dietician was going over starches, she didn't focus on eliminating white processed flour, but I will. All the processed grain, even most whole wheat is processed, with a little wheat to make it brown. Sprouted whole grain is best, and if u can, cut out sugars completely. That's the most important thing. U can have artificial sweetener if u must have it-
For proof- my 1 hr test came back 17 points high and my 2 hr test (I'm on strict bed rest) came back about 90 points over. I stopped drinking fruit juice, sugar, and minimized my intake of white flour, and now my readings are normal as of now. I can't exercise, only stretches, but if u can exercise regularly. It's huge in helping regulate blood sugars. Best wishes!