Oh you’re right, Pedro, gown is still used a lot especially in print as you point out. I misspoke when I said it’s no longer used. People do use it when speaking too, I’m sure. What I should have said is *I* don’t hear it used much anymore. Girls I know refer to their vestido de formatura as prom dress and brides I know refer to their vestido de noiva as their wedding dress. This doesn't mean it’s wrong to say or write “prom gown” or “wedding gown.” It’s all the same. The word “gown” as in ball gown, night gown and evening gown is starting to sound a bit archaic as those garments are less commonly worn and so the word is becoming old-fashioned.
Gown is a perfectly good, descriptive word to mean a long dress!