I got diagnosed with GD at 29 weeks and until then I was putting on weight quiet quickly, I was upto 13kgs already making me 71kgs (I'm 152cm tall) I then had to eat as a diabetic and lost 2kgs in the first 2 weeks and have stayed the same (69kg) for 7 weeks. What I'm trying to say is it's perfectly healthy to eat a low fat, low sugar, low GI diet while pregnant although it's not easy it is possible. I would see a gp and a nutritionist aswell if you can just to help you get the support and guidence but I'm sure you can do it