I don't know enough about breastfeeding with pre-eclampsia but I can answer about GDM.
If you have true GDM (and this is not Type 2 diabetes that just happens to have presented during pregnancy), your glucose will go back to normal within minutes of delivering the placenta. Your diabetes team will be able to tell you whether they want you to stop all treatment, which is what we do in most cases, or significantly reduce treatment.
Assuming you are on no treatment, you can carry on with breastfeeding with no special considerations. If you are on treatment, remember that your blood sugars don't need to be as tight as before and it is far better to run higher for the next few days and weeks than it is to run low and end up hypo with a little baby.
In terms of the baby, as I am sure your team will have told you, there is a risk of hypoglycaemia for the baby in the hours after birth. This is because the baby will have produced more insulin than normal to deal with your slightly higher blood sugars so it is really important to keep your blood sugars as normal as possible even up to the birth.
Hopefully you'll get help to breastfeed and it will all be fine. Sometimes, however, with all the best will in the world, babies do need a little bit of expressed milk or formula to raise their blood sugars so they have enough energy to latch on and feed properly. This is not a disaster and does not mean the end of breastfeeding, think of it as a medicine to help things, so that the baby can get on with breastfeeding properly feeling a bit more with it! 
Hope you are not getting too bored in hospital and that all is well with you and your baby.