Just eat what you would normally eat
The test is to test how your body breaks things down - so by fasting for 12 hours it gives your body a chance to break down sugars and expel them. The test will pick up if your body is/isn't coping.
The GD test is a standard test during pregnancy, so it's nothing that you're "made" to do based on your pee-on-a-stick test results at each MW/OB visit. It's a precautionary test, more than anything
Good luck - take a book/magazine to read while you wait