Go to webmd and learn more about gestational diabetes.
Gestational diabetes can happen in all types of women - healthy, lean, obese, sedentary, active, etc. The pregnancy hormones interfere with the body's ability to make insulin, not your genes or your lifestyle (although being overweight, inactive, and having a poor diet can contribute to type 2 diabetes).
I took two 3-hour tests. Just make sure you bring a lot of reading material because you'll be there literally for 3 hours. Do it first thing in the morning too, because you'll need to fast. You'll do a blood draw then drink/eat the glucose and have blood draws one, two, and three hours later.
Although I was never formally diagnosed with GD, I was at the highest acceptable level so my doctor wanted me to follow a GD diet. I did that for 2 months, took the 3-hour again, and got the same exact numbers - go figure! On the bright side, my baby was born at 37 weeks, 7 pounds 1 ounce (no macrosmia!), and I delivered vaginally.
Go to reputable websites and read up on GD. You'll really learn a lot.