What you eat CANNOT cause GD, so please don't feel guilty. It can though make it worse, so changing your diet a bit could well improve things in a week.
The main idea is to avoid sugar. Not just sugar in sweet things but also the sugar in simple carbs - that's white bread, white rice, etc. Try to eat whole meal versions of carbohydrates, and in fairly limited amounts. Different people react to different things - eg I couldn't really tolerate bread at all but could cope with potato fairly well. If you are diagnosed with GD you'll be asked to monitor your blood sugar (again, very straightforward once you've done it a few times) and you'll find out through trial and error what affects you. Avoid fruit juice too - better to eat the actual fruit, and keep to the lower sugar fruits like apples and berries. As a rule of thumb, if it's native to the UK, it will be lower in sugar.
Another thing to do is to always combine protein with your carbs, as that slows the sugar release. Ditto fats too, so cheese on an oatcake makes a great snack, or hummus. You can also have occasional treats! Exercise is great for lowering blood sugar too, even just walking or housework.
There are lots of resources out there. As TinyMonkey says, google low-GI diets for ideas. Millions of people eat like this all the time without feeing deprived (I'm one of them) so you WILL cope if you need to do this. And if you can control your blood sugars this way, you minimise any potential effects of GD. I had a very normal sized baby with no health issues at all.