Hi,
It's rotten to have gastro when you have such a young baby. Many mothers feel like they won't have enough milk. but, interestingly, mothers make milk when they are sick the same as any other time - but when you are so sick it makes you see things differently. Have you got some-one to help you there and take care of your baby? You will really need the support. If you can manage it, keep breastfeeding - you will have enough milk. It is not the sickness that will reduce your milk supply, but the formula filling your baby so he doesn't need to feed enough. Your body is already making antibodies to protect your baby from getting the same illness you have. (These live cells can be detected by researchers in human milk even before the mum knows she is sick) Formula doesn't have these cells to protect your baby.
Hope you are feeling better soon