Hi Penny12355,
Sometimes getting the news you have GD can be hard to accept. I was upset when I was diagnosed at 28 weeks and felt it was my fault but contrary to what you may think having a high BMI doesn't automatically mean you will get GD - there are a number of risk factors but these don't mean if you don't have BMI that's high you don't have it. It's a condition caused by the hormones in the body which make it insulin resistant.
If you fasted for 12 hours before the GTT then the test has been administered correctly and your results will not have been impacted by the food you ate.
You will be seen by the diabetic team who will advise you on acceptable test targets for fasting and post-meal bloods (different NHS trusts may have different targets) so when you say your sugar levels have been good if you mean then they have been within target then that is great however it doesn't mean that you don't have GD.
With the testing of blood sugars you can provide the diabetic midwife/consultant with an accurate picture of how well your body is processing sugars - it's a progressive condition which means it gets harder to control your sugars as your pregnancy progresses so you will need to monitor to keep an eye on this.
If you follow the GD diet that you will be told about - my hospital invited me to a session with a dietician and diabetic midwife (there are also plenty of helpful resources online) you should be able to continue to keep your sugars within range. It's a lot of trial and error and some of the things they said would be ok to eat were not ok for me. Everyone is different.
Unfortunately whilst the diet controlled my sugars during the day my fasting sugars first thing in the morning were too high (nothing to do with what I was eating and actually very hard to control) so I am now on medication for this.
Yes you will have to have more scans and more appointments but with GD comes the risk of a larger baby (my first growth scan showed my baby was big but this has reduced since I got my diagnosis and began managing the condition) and also many other serious complications - many of which you can read about online - but that necessitate closer monitoring of you and baby in pregnancy to make sure you are both safe!
If you have been told that you have GD - even if you don't agree - you need to follow the guidance you have been given to avoid any risk to you and baby!