Yes. 100% polyester gowns with no accessories can actually be machine washed per the label instructions.
TIP: The first time you clean any gown, it should be dry cleaned to make sure all colors are set.
Should your gown be machine washable, in general follow these directions: Use mild soap at a warm setting. Tumble dry at a low heat or hang dry after a complete spin cycle.
Upon receipt of your graduation gown, Jostens advises their customers to promptly remove it from the packaging and hang it up. To remove wrinkles, you may choose to press with a cool iron, or hang gown in a steamy room.
Of interet, check out RealMenRealStyle.com's Tips on How to Clean Your Delicate & Expensive Clothing.