Sorry this has happened try not to panic, it is manageable, I had GD with my first, not with my second and then discovered I had Type2 with my third, the main thing is to try and maintain a good blood sugar level, things that make it go too high are refined carbs, sugars and even some fruits.
A good place to start is to avoid or limit things like.
White Bread, Burgen bread is the best or Rye.
Cakes, biscuits, sweets, sweetened yogurt, ice cream, tarts, chocolates, dark chocolate 70% and up is better.
Pancakes, Yorkshire puds, potatoes, boiled new or baby potatoes are best.
Pastry, any pudding/desert really.
Low sugar jelly and cream is a good desert.
Obviously and fizzy drinks full sugar.
Crisps, packet sauces that use white flour.
Rice is not good for blood sugar, quinoa or lentils are better.
To give you an idea my typical daily menu,
Breakfast: 2 poached eggs, 2 rashers of grilled bacon, 2 flat mushrooms, one grilled tomato, one slice of burgen bread with real unsalted butter, hot black tea/coffee with one sweetener.
Lunch: lettuce, cucumber, red onion, one chicken breast grilled, sliced, some ham or bacon, blue cheese dressing I make myself.
Or
Tuna, spinach, boiled egg quarters, mayo.
Dinner is usually whatever the family is having but minus or a small portion of carbs.
Roast chicken thigh, palm serving of potato salad, beetroot, salad or fish baked in foil and butter with some lemon/herbs child's serving of potatoes/rice, plenty of veg.
If I want something sweet I have Greek yogurt (real Greek yogurt) mixed with a teaspoon of honey (very little) with chopped nuts or just berries and half a banana and no honey.
I use almond milk in tea if I want it white or heavy cream in coffee as normal milk can spike blood sugars if you drink a lot of tea/coffee.
It's also helpful to keep blood sugars down to eat a good amount of fat with any carbs you have, it slows down how quickly the carbs turn into sugar when digesting which reduces a spike.
With my first I had GD from 28 weeks and baby was born at 39 weeks weighing in at 7lbs 9oz even though the growth scans were saying 8lbs+ from 33 weeks so they have a large margin for error.