With my first, I failed both my 1 and 3 hr test and was diagnosed with GD. With my second, I passed my 1 hr test, no GD (which, apparently, is weird because if you have GD the first pregnancy you have a 80%+ chance of having it with your second).
For my first, my OB mentioned that I shouldn't restrict carbs in my diet (maybe some women try and restrict them in order to pass the test?) and have a little extra carbs for a few days prior. I think he actually said, indulge yourself and have a candy bar after dinner for a few days (no, really!).
For my second, I did NOT do the extra carbs and passed.
I really think that it's just up to your body type/metabolism. I'm not 100% sure I really had GD with my first pregnancy. Sure, my blood sugar fell within the 'range' for GD during the test, but I think I'm just slow to process carbs. One of the big concerns of GD is having a large baby, and my full term kiddo was only 6lbs 7 oz!
That being said, please don't try to "rig" your eating just to pass the test. if you do in fact have a great likelihood for GD, you want to make sure you and your baby are being treated for it. All I ended up doing was monitoring my blood sugar and restricting my carbs a bit. Definitely not a big deal....