To explain a little of why they do the high carb before the test:

They are trying to see how your pancrease responds to being put under stress. When you are pregnant and in the last 15 weeks or so you require anything from 2 - 4 times the normal amounts of insulin to process what you are eating. It is to do with all the other hormones affecting how well the insulin is used. GD occurs when your pancreas can't handle that kind of stress load and is not managing to produce enough insulin to metabolise the glucose effectively. By giving you a carb loaded diet for a few days before and then maxing it out with the GTT drink they can get a rough idea of how well your pancreas is performing under stress. If you go low carb before you may get a false negative result. If you then thought Gee I am okay (but actually you have GD but it just didnt show up in that one test) and go back to normal high carb diet you could be doing harm to your body.
On the other hand If you go high carb and get a result just over the limit like the other poster said or get a false positive the worst case is that you will get to do some finger ***** tests for a couple of weeks initially to make sure. If your sugars are all good over a period of time then you can prolly relax a bit more.
Ideally the best way to diagnose GD would be to give every pregnant women a blood testing machine and get them to test several times a day for 2 - 3 weeks as this is much more accurate as you get to see a pattern of how your body is working under everyday circumstances. Unfortunately it is just not practical..... can you imagine the compliance issues....no one would want to do that voluntarily, hence the gross carb loaded GTT test.