I was measuring 37cm at 34 weeks (turned out to be constipation, not a big baby). I'm a size 8, so the bump always looks big on me, but was still sent for a growth scan and GTT.
Growth scan was fine, but I failed the GTT (borderline result) at 36 weeks (now 37+6) and have been self blood glucose monitoring. It's not so bad, I would rather they stick needles in me prior to the birth, than stick needles in the baby afterwards.
The only change to my diet has been no fruit juice in the morning, asides that I am eating as usual with no raised glucose levels without any medication.
Try not to worry too much, but defo have the scan, it puts everyone's minds at rest when they see a normal growth curve, especially if you do turn out to have GD, as they will start talking about early inductions etc, if the growth looks fine, then it's easier to stand your ground on your birth choices.
You can ask the sonographer to keep the scan as quick as possible, so as not to distress the baby too much - they only need to measure the head, tummy, kidneys and check how much fluid is present, so it can take just a few moments.
Good luck with everything, try to remember not all fruit ripens at the same time and don't be pressured/bullied into decisions that you feel uncomfortable with, don't be afraid to ask for second opinions, the best thing for you and your baby is for you to be happy and confident in your choices, whatever they may be.