i have type 1 diabetes, so I know a little about this.
contrary to popular belief, diabetes isnt about sugar, its a disease of carbohydrate metabolism.
all carbohydrates in food (whether that be bread, pasta, brown rice, white rice, mars bars, wine gums, orange juice) are broken down into simple sugars during the process of digestion, and in simple terms, the hormone insulin allows the end product of carbohydrate metabolism, glucose, to be taken into the cells and used as energy, rather than floating around wreaking havoc as 'high blood sugar'.
in type 1 diabetes, the body produces no insulin, whereas type 2 is generally where your body becomes very resistant to insulin, and it needs a lot more insulin to get that glucose into the body's cells. in pregnancy we become naturally resistant to insulin (so for me, i started my pregnancy taking approximately 50 units of insulin per day, at 31 weeks this is close to 150 to do the same job, lucky for me i dont have to worry about whether my pancreas can keep up with the increased demand, i know it cant and just adjust my doses gradually as a result of testing my blood sugar frequently). for some people, this results in GD: ie the body is unable to produce enough insulin to keep the blood sugar level in a normal range.
The GTT is simply the best way of testing your body's insulin response. It makes no difference what you are used to eating, your body should respond to the presence of rising blood sugar levels by releasing insulin; the only difference in a diet predominantly consisting of low GI foods is that the insulin response is slower as blood sugar levels elevate more gradually - but this doesnt mean it doesnt quicken up when presented with sugary drinks.
If the results come back as impaired, this indicates that you have an impaired insulin response. In a mild case it can be controlled by reducing the carb's in one's diet, and by sticking to low GI carbs like brown rice, wholegrains etc: as I mentioned in the previous paragraph this means that the food is broken down more slowly into glucose giving your body more time to release adequate insulin. However, sometimes this is not enough and your body is temporariily in a position where it is unable to effectively metabolise even moderate amounts of low GI carbs, and in this case you need to take insulin for the remainder of your pregnancy. So however healthy your diet, it's important to have the test as it doesnt mean you cant develop the condition or neessarily control it....
Anyhow, try not to worry about this test, the point is, the results are completely independent of the kind of diet you have and even with the most healthy diet, insulin can be required, so try not to fret, have the test, and then come back and let us know how you get on.