I'm 29yr Old Pregnant With 1st Child (30wks) With No

I'm 29yr old pregnant with 1st child (30wks) with no family history of diabetes. I took a GTT at 26weeks and the results after the third reading (2hrs after drinking the glucose drink) was 131. At the time My OBGYN said I should watch my carbs and sugar intake because thats high. I have been doing so ever since, and today I saw my OBGYN and she said the baby size is fine but since I gained 4kgs since last visit that I have GDM. Is that possible to tell just from 2kgs over expected weight gain?