Don't feel bad. I was shocked to find I had GD - I am slim (early pregnancy BMI 18), do sport and eat healthily. Sometimes the pregnant body just goes a bit mad. I think my risk was mainly genetic as my dad has Type II diabetes.
What does it mean?
Initially they'll probably want to control your sugar levels through diet only. If this isn't working then they will look at a safe drug like Metformin, and if this doesn't work will look at injectable insulin.
Buy this book (also an app) - www.carbsandcals.com/ recommended by my gestational diabetes nurse. An eye opener for me was the portion control of carbs. I am used to eating massive bowls of wholewheat pasta. rice after sport and large bowls of muesli and hat to cut these down. Also interesting in that chocolate eclairs, if I remember correctly, are low carb! Obviously they are also high in fat and high fat diets can affect how your body measure insulin (IIRC), but I needed fattening up at that stage.
They'll give you an electronic blood sugar monitor with test strips and you will need to test before and 2 hrs after 2-3 meals a day. You keep a food diary and soon learn what will take you over the very strict limit (stricter than in normal diabetes I think due to the risk to the baby). I found a latte could take me to the limit, so latte and a sandwich was a no no due to the carbs. You will get sore fingers from the testing.
You will have regular - maybe fortnightly - check ups at the hospital as you will prob be classed as high risk, but don't panic too much due to that label. They'll also likely do some late growth scans and also check your amniotic fluid levels, as a further complication is polyhydramnios (excess fluid), which can be risky for the baby. Take your partner to scans as they can find complications and want to make decisions.
If your levels are well controlled and baby seems ok then they'll probably induce you at about 38/39 weeks. If you are on insulin I believe they induce at 37 and may give the baby steroids for its lungs.
After birth the risk is that the baby might have a sugar low which can be dangerous as its body is used to higher sugar levels. Therefore it will be frequently monitored for blood sugar and could spend time on Special Care Baby Unit. It might need a glucose drip and will need early breast/bottle feed.
IIRC GD can delay milk coming in so some MW advise priming your breasts and actually squeezing out some colostrum in the few days before birth and freezing it in little syringes they'll give you. It can be quite sore, but even a few drops are good (and are all you can expect at first). Take these to hospital with you, make sure they're frozen with your name etc on and get the staff to feed the baby with that as they're not always effective suckers at first and if their blood sugar levels are dropping the staff may want to supplement with glucose or a formula feed. Usually the baby is fine, often they don't need anything (the better controlled your sugars in the run up to the birth the more likely this is).
You will probably have weird blood sugar levels for about 48 hrs and then things should usually be normal. At your 6 week check you will need to do another GTT and may need to do a GTT every year for life now.
So basically - carbs and obvious sugars bad, exercise and protein good. So eggs, nuts, seeds, tofu, meat, fish, cheese all good. Pasta, rice, bread etc not so good :-( As a vegetarian with hyperemesis who could only eat lucozade due to the hyperemesis, I was stuffed but did manage to keep my sugars controlled (by diet alone). I lost even more weight though.
For me my baby was delivered by c-section at 38 weeks due to his large size (8.10 at 38 weeks), severe polyhydramnios and a number of other serious complications I had by then. He had respiratory problems after birth, which I think can be a consequence of polyhydramnios, which is a GD complication, and was in NICU on a ventilator for 2 days, but it now seems there may be other unrelated issues that could account for that. He did have a slight bloody sugar dip IIRC, and was fed formula as I wasn't producing anything and due to his lung damage the staff needed to stop him screaming quickly. We did manage to EBF for about 6 weeks.
At my 6 week check the GTT showed my pancreas isn't working properly and I am borderline diabetic.
I think that's all, hope that was helpful.