I don't know much about GD, but I know a lot about Type 1 and 2 diabetes. I'm assuming that the concept, with the exception of insulin production, is pretty much the same. If you (or anyone) thinks that they might be at risk of failing the 1 or 3 hour test, my suggestion would be to eat as few carbohydrates, especially ones that are super high in fast acting sugars (juice, candy, soda, etc). If you eat high sugar/carb meal that is also high in fat, the fat inhibits the absorbtion of glucose in the blood initially, and you can in theory have a blood sugar spike after a couple of hours. This would happen with something like pizza, for example. Fat/protein slows down sugar - so things like peanut butter and chocolate will be high and sugar, but won't cause an immediate BG reading spike.
I hope that makes sense and doesn't confuse people. Basically if you're worried, eat a lot of vegetables 24 hours and leave out a lot of carbs the day before your test. :)