I had GD and got told in an nct class to try and express some colostrum
When I told this to my midwife she was very
and said it really wasn't necessary and to trust my body.
After birth my colostrum increased 10 fold and I am sitting here typing this wearily breast feeding him now
If you REALLY want to do this then go to boots and buy a 5 or 10 ml syringe, sterilise and when you squeeze a drop out suck it up into the syringe
You should then freeze the syringe and will need to take from freezer to hospital well labeled and ask them to store for you in a fridge
It really stressed me out trying to do this which is the last thing you need in late pregnancy so I'm glad my midwife told me not to
I can understand why you're worried though, I'm not trying to be dismissive.
After birth they will test your baby's blood sugar before the first two feeds and it has to be above 2.6 I believe
My DS was 3.1 both times and wasn't exactly great at feeding initially (now he's a milk monster)
Have a little faith in your body, plus if you need some help the midwife will help you express some colostrum after birth which is much easier because they are more skilled at it and you have more!