Don't panic!
I went for the test at 13 weeks, but threw up throughout it, so was retested at 17 weeks, which is still much earlier than most people get tested (my test was due to family history and being overweight). I tested positive unfortunately, but have managed to control my GD through diet, and am due to give birth next week .
The GTT means fasting for 12 hours, then having blood taken, being given a glucose drink, sitting around for 2 hours and then having another blood test. They will check to see how much sugar was in your blood after fasting, and whether your body has been able to deal with the input of glucose sufficiently.
If you do test positive, there is a small chance that you may have been type 2 diabetic before the pregnancy, but a few days of monitoring your own blood sugar at home (finger prick tests which you'd normally do 4 times a day) should be enough for the consultant to decide whether it is type 2 or just gestational.
There is a support thread on here for those of us with GD, so come and join us if your results are positive.