En klänning = a dress (en-word)
Klänningen = the dress
Klänningar = dresses
Klänningarna = the dresses

Sadly, these are not interchangeable.
https://www.duolingo.com/skill/sv/Definite-forms/tips-and-notes
https://www.duolingo.com/skill/sv/Plurals/tips-and-notes

Both are acceptable. They're pretty much interchangeable except "gillar" might be a teensy bit more casual, which is probably why a lot of Swedes told you that's what they use.