Hun a high GCT result means nothing the big M could have been an infuence.

Diet and overweight can contribute to it but does not mean you will get GDM, I have a friend who was a tiny stick of a girl and would never eat lollies, soft drink, junk food etc and she had GDM.

Just follow instructions for 2 hrs GTT and wait and see if it comes back that you do have GDM then hopefully you can control with diet alone. As many people do successfully.

Good luck and rememmber it is not the worst thing in the world you can go through.