Is there effective medication that will decrease gastroenteritis symptoms?

Yes. You do not specify your symptoms, but there are numerous medications available for your symptoms.
Yes many. There are many great medications that can decrease gastroenteritis. Acid buffers have been around for ever and can be effective.(maalox/mylanta/tums). Histamine blockers (zantac or ranitidine/ Pepcid or famotidine) are very effective at reducing acid and therefore helping with symptoms. And the newer proton pump inhibitors such as Prilosec (omeprazole) and Prevacid (lansoprazole) are wonderful at controlling.