I had GD with my first and this time, I requested immediate diagnosisnand am checking my sugars routinely but not being tested formally for it. I had no risk factors for it either (25 with the first, 5'3" and 110 lbs or so both times) - sometimes the hormones just cause it to happen!
I failed mine by one point with my first baby, and then failed the three hour by a few points at the one and two hour marker. With a level above 200 for the one hour, my doctor's protocol is to diagnose immediately and skip the 3 hour altogether.
That being said, GD is not the end of the world, and hardly poses any risk to the baby if it can be diet controlled (my OB went as far as to say my case of it poses no risk at all). Both of mine have been easily diet controlled with zero issues. Even if you end up needing insulin or other medication, the risks of GD are minimized greatly by keeping on top of your diet/exercise and taking needed meds so blood sugar can be controlled. Sometimes women with more severe cases of GD are required to have more ultrasounds or go in for non stress tests, however none of that was ever recommended to me.
If you do end up being diagnosed, your doctor will give you tips on your diet or may refer you to a dietician. You may be told to eat small meals and snacks throughout the day, as opposed to large meals. You will need to limit (but not eliminate) carbs and restrict refined sugar intake. I was initially given carb counts to aim for per snack and per meal. You'll be prescribed a glucose meter so that you can check your blood sugar as frequently as your doctors suggest. You may be told to take walks or exercise after meals, as this can really help keep your numbers in check. It might affect your c-section date or it might not... I can't speak to that at all, as I went into labor and had a vaginal delivery. The biggest fear with GD is the baby becoming too big, however your doctors will be able to monitor that as needed.
No matter what happens, don't feel guilty and try not too worry to much. There is not much you can do to prevent GD - it's just all those hormones wreaking havoc! Hopefully you will pass your three hour and not have to think about it again!