ultrasound measurements can be off by around 20% as it depends on what the gynae can see and measure at that point of time. but since you have GDM, it'll be best to follow your gynae's advice for the sake of you and bb, even if it means you can't deliver naturally in the end. Just to check, how heavy is your bb at week 29?
I had gdm and gynae induced labor at week 39. Normal delivery with epidural. All went well. Although gynae worried baby was on the bigger size, baby came out normal weight.
Most of us want to have normal delivery including me but just remember to be flexible when circumstances tell you otherwise