I had gastro a couple of months ago and could not keep anything down either. My Doctor said just to have small sips of hydralyte between throwing up. I was throwing most of it back up but he said some would be getting obsorbed straight away. He said try not to feed gastro and just let it pass. So I had nothing at all besides the hydralyte for 24hours.
DD2 BF a little more frequent during this time but we were both back to normal in a few days.

Gastro sucks and I hope you start feeling better real soon.