I’m all for modesty but that is not a “gown”. The Mormons get modest dresses right: http://www.jenclothing.com/church-dresses.html even if that is the only thing they get right.
Yes, it is a gown. People all over the world wear gowns, often in styles that have persisted for centuries because they are practical and attractive.
The Mormon young ladies’ dresses are nice, too. I wear similar styles many Sundays, although I like a longer skirt.