I don't know if i had GASTRO, but it was "loose bowels" lol. At the chemist. I went to doctor first and they gave me stupid rehydration medicine, which made me spew, yes seirously (i was practcially eating powder which sucked up the glas of wate, I felt much worse, and it was salty flavour). So don;t bother buying those, no point, I just drank water.
Um, just ask at the chemist for ANYTHING, they are all bound to work. Also avoid EVERY food, just seriously eat bread,bread,bread.
But I forgot to mention, if you have a parasite, which I think Gastro is, you need to "poop" (sorry), for a little while to ensure it is cleansed from your body, if you take tablets and stop yourself from going toilet it remains trapped inside, so yeah.
Sorry I can't remember exact name, but try "ethical nutrients" I see their advertisements all the time on television, so at leats its a well known brand and not some random.
Goodluck again!