Deepa - you and I seem to be having some similar problems
I failed 1 of the 4 results (fasting, 2 hour & 3 hour were okay, but 1 hour was too high.) They said I'm "glucose intolerant" not gestational diabetes, but the result is the same, I need to watch my diet.
Basically, your placental hormones block the pancreas from creating enough insulin to process the glucose in your blood in a timely manner. The risks of GD are primarily around having a baby that is too big. It's more common with pregnant ladies over 35.
I met with the diabetes RN and she gave me diet tips as well as a blood sugar monitor which I am to use 4x per day (first thing in the morning and then 1 hour after every meal.) Goal is fasting less than 90 and 1 hour post meal target is between 100-130.
In terms of diet, the nurse explained to me that the key is not sugar, but carbohydrates. You'll learn to read the sides of every food containing carbs. Take the total carbs and subtract any fiber (so like if a piece of bread is 20 grams of carb and 2 grams of fiber the net carb is 18 grams.) I was told to have 30 grams of carb at each meal and then two snacks of about 15 grams of carb, so a total of about 120 carbs per day. The key is to even out your carbs through the day so you don't have too much at once. Proteins and fats are 'free' so you can eat as much of those as you want - in fact I've had to increase my fats because I started losing weight. Also, eating fats with carbs helps to slow down the release of the sugar in the blood and helps your pancreas keep up, so if you eat carbs, try to eat fats with it (nuts and peanut butter will be your best friend!)
After a while, you'll probably figure out what foods raise your sugars more than others. For example, I can't eat cereal w/ milk in the morning anymore as it really raises my sugars in the morning, so I'm eating mostly eggs and toast now for breakfast (eggs will be another food you'll eat a lot

) And, mornings are my hardest, since that's when the placenta hormones are at their peak. Later in the day, I seem to be able to eat almost what I want and not have a problem. After about a week, you'll know what to do!
It is hard for us w/ previa also since that means we can't exercise (well, now I'm on bed rest so I definitely can't do anything.) But, hopefully you can control it with diet. I've had no problem thus far doing that by following the diet. And, if not, insulin shots will be minor compared to the stims and PIO we've had to endure - you'll have no problem if it comes to that.
I've also found this resource which has been helpful...
http://www.pregnancy.org/phpBB2/viewforum.php?f=31
Good luck!