Well managed GD isn't the panick inducing thing you seem to have got into your head.
Firstly you're boarderline so you will probably be able to manage your GD with a few changes in diet and monitoring your bloods at home. Most people can.
With any condition there are risks, but worse case senarios aren't going to do you any good at this point. I'm picky about my sources and have never linked birth defects as a high risk to well managed GD. To be honest at 28 weeks you baby is maturing rather than developing.
My experience is being diagnosed at 28 weeks making changes to my diet, more regular scans to check the babys growth and the condition of placenta. I was allowed to go full term and delivered a healthy 7lb7oz DD without any medical intervention.
You will be monitored more closely, more scans etc. as I undrstand it the risk is more that your baby will get too big which may cause complications with the birth- so you need to keep sugar levels balanced and will be supported in doing this medically if you can't do it by diet alone. Also they scan you to check the placenta, as there is a risk that it starts to break down at the very end of the pregnancy. Both of these things they look for and will induce at 38 weeks if they have any concerns at all.
If it helps a friend of mine had GD from the word go with all three of her pregnancies, all was fine and she has three strapping lads to prove it.
As Sh77 says you biggest risk is a bigger baby. And you probably can't eat cake anymore.
On the plus side my sister bought me the biggest box of Thorntons Continentals into the hospital after DD was born, chocolate has never tasted so good!
Be cool and take good advice.