I think that you are asking about the oral glucose tolerance test (glucose challenge test), because the tests for diabetes and gestational diabetes are quite identical.
However, in purpose to be sure that you are talking about this, I will mention that during the glucose challenge test, you are given a glucose drink and after you drink it a blood sample is taken from you.
Anyways, regarding the results, you have to know that everything depends on the quality of the laboratory.
Generally such thing doesn’t take more than 2 hours or even less. However, some laboratories, are not working so fast, due to many reasons, which means that you may probably wait for about a day or two.
Besides this, I want to remind you that the results are not the most important thing. Your doctor will need time to look and them and to think of your eventual problem and its solution.
I mean that after you get the results you have to talk with your healthcare provider about them. He/she will tell you the true about your condition.