If my roommate possibly has gastroenteritis and threw up in our bathroom can I shower without getting sick?

Do good handwashing. Gastro is generally passed by the oral secretions of the infected person. The germ can live for hours on door knobs or other surfaces they touch after they have touched their face or mouth. Avoid contaminated surfaces when you can & practice frequent hand washing. Don't share towels or other linen or drink from the same glass.You can shower in the same br if you follow these rules.
Dirty Shower. Ok to shower, but dont' drink the drain water. Gastroenteritis is most commonly transferred by ingesting the virus in fluids. Casual contact in the shower should not be a problem.