When choosing carbs for your diet, GI is important but not as important as quality. The best carbs are natural, unprocessed carbs, and you should focus mostly on the complex variety. GI is only a secondary consideration.
If you relied purely on GI for choosing your carbs, you would choose Coca Cola (GI 63) over watermelon (GI 72) or a Mars Bar (GI 68) over a baked potato (GI 85).
The GI of a carb means less in the real world than you think. GI values of carbs are measured by test subjects eating the carb on it’s own, on an empty stomach, and in a portion size that provides 50g of carbohydrate. In the case of watermelon, that’s an 800g serving, or for carrots, a serving of 820g – both not very realistic is practice.
In the real world you usually don’t eat on a totally empty stomach, especially if you’re having frequent small meals each day. Undigested food in your system actually lowers the GI of any carbs you eat. You also don’t eat carbs on their own (at least, you shouldn’t be). Whenever you have proteins, fats, and fiber with your carbs, these also lower the carb’s GI.
So by eating small, regular meals with sensible portion sizes, and having carbs only as part of balanced meals, the overall GI of your carbs is much smaller than the tables state and so GI doesn’t have as big an effect as you might think!