You are not required to fast prior to the 1 hour glucose tolerance test but after you’ve drunk the glucose solution you won’t be able to eat or drink anything till your blood is taken. There are women who say that eating before taking the test has usually made them fail and some women have said they did not have anything to eat and still failed the first test but go on to pass the 2nd, 3 hour glucose tolerance test. Typically it’s not recommended to eat anything 2 to 3 hours before the test and anything very high in sugar. If you must eat, choosing something high in protein is recommended.

See:
Gestational Diabetes Testing
1 Hour Glucose Test
3 Hour Glucose Tolerance Test

The 3 hour glucose tolerance test requires that you fast for 12 to 14 hours earlier since the test starts with a “fasting base glucose” to compare the rest of the blood draws after you have drunk the glucose solution. You won’t be able to eat anything between blood draws for this test which is taken every hour, 3 more times.

More Information About Gestational Diabetes
Gestational Diabetes Diet – Gestational Diabetes and Blood Sugar Levels
Carb Counting and Gestational Diabetes – Gestational Diabetes After the Baby