I hardly gained any weight when I was diagnosed with gd and infact I lost weight. No one was ever concerned about it tho as my fundal measurements were normal.
I was just diet controlled and my advice to you is to eat low gi foods and more often. I didnt do just the three main meals per day. I had about 6 and they were small portions and always low gi foods. As I got further along my levels didn't spike as they said they would and I think it was due to what and how I was eating! No more than 3-4 serves of carbs per meal and make them low gi carb foods were possible. Basmati rice is better than most rices, wholemeal multigrain breads, smaller portions of pastas and cereals...
Not sure why they said insulin will cause a bigger baby as that's simply not true. It's all the sugar your body can't control that passes to bubs.
My bubs was born 3.77kg which is an average size and his blood sugars were tested after birth about four times and he was fine.
Also I did at least a half hour walk each day and exercise will keep levels stable.