First...relax. GD is harder to control when you are stressed. Stress, diet, excercise all contibrute to your levels.
If your first test (the one hour test) came back high, you may not have GD. My 1 hr test came back high w/ both pregnancies but only with the second pregnancy did I have GD.
If you have already had the 3 hr test and it too came back high then you will most likely be meeting w/ a nutritionist/dietician. You will also have to begin checking your levels (and the guidelines are different for GD then they are for regular diabetes) and try to keep it in check. Depending on your diet/eating habits, you may not have to change much. The best thing is to listen to what they tell you and try to regulate you levels through diet. If that doesn't work, you will probably have to start w/ insulin (there are different types and what type you need would depend on what your numbers are and how your body processes the foods you eat).
I was told that they normally try to induce around 38 weeks to avoid the baby being too big and having complications from the GD. My due date "snuck up" on my doctor and I was scheduled to be induced just about 4 days early. I went in labor on my own two days before that, so I was just over 39 weeks. My daughter was just 6 lbs 11 oz (not too big and no problems/my son was 7 lbs 15.5 oz and I didn't have GD w/ him). Neither of us have diabetes now.
Things will work out fine. Follow the dietician and doctor's advice. Try to control the GD through diet but don't stress if insulin is needed. The further along you get in your pregnancy the harder it to control your levels. I had the worse time w/ my numbers after lunch even after I was put in insulin.
In an effort to make sure you understand that this is important and take it seriously, they tend to scare you a bit. If your numbers are just a point or two high, it is not the end for the world. It is important that you watch what you eat and follow their advice but the most imporant part is to not let the numbers get too low or too high. Be sure you eat your evening snack (they will advise you) to help avoid being too low in the morning since you will be going a long period without eating and if you have taken insulin, be sure you eat.
You will look at food differently...some vegetables are sugars, some are better to load up on. Sometime cheese may be your dairy but sometimes it can be your protein....they don't always fall under the traditional food group that we learned them to be in. Portion control is key....learn what you can have extra of without causing your levels to spike for those times you are hungrier than normal. Also, if you are really craving something, you are better off to have a small portion of it then try to resist and then pig out on it.