weight is not the only cause of GD it contributes but whats done is done. I had GD too - what does it mean, monitoring bloods (finger prick) and using a meter and recording results, eating sensibly (note I do not say ban all sweet things!!!) you have to be realistic about what you are eating and yes you will need to think about what you are eating a bit more - so whole or half whole pasta, basmati or brown rice, brown bread etc plus the usual veg and protein.
Tips and tricks - if you are going to have something sweeter have it as part of your meal rather than a snack (The meal helps balance overall blood glucose levels) stick to healthier sweet things rather than pure sugar. Always carry some sort of healthy snack and something like jellybabies in case you have a low by getting insulin wrong etc - try and test before taking so you are sure its a low though. Get a larger handbag for the testing pack and the snacks!!!
Caffeine can also effect blood sugars as can some sweetners, I found sweetners and e.g. xero sugar pop really didn't help - stick to decaf tea and water NOT Milk which has lots of lactose
Look into GI eating or the blood sugar diet or Zoe Harcombe. Learn how it all works and use the maintain version of these diets rather than the lose part. You will likely lose anyway - I lost 4st while pregnant and I only knew I was pregnant for around 18 weeks as she arrived early (I am much older as well and had gallstone issues too) and I didn't know I was pregnant for ages.
If you are smart with the learning and understanding the diabetic nurse you are assigned will help you calculate the insulin you need and the types and this gives you much more flexibility in what you can eat.
What I learnt I didn't know was when you are pregnant you need approx 4 x the normal amount of insulin in your body. I was gobsmacked.
In essence don't panic, you can't change what has happened but you can deal with it as best you can. Please also be aware that any sort of infection will also send your blood sugars bonkers so take care of any cuts and grazes properly, get your teeth checked and any work done thats needed, look after your feet :-)