"Grandfather is wearing a dress."
Translation:할아버님께서 원피스를 입고 계세요.
The term "trans" is generally used to talk about transgender or transexual people. Dresses are usually regarded as female pieces of clothing, although they're just cloth. Based on this, a Grandfather (a male) wearing a dress, would be a transvestite, not a transgender person. Reducing people to adjectives is really offensive, it would be the same as saying "a fat", "a black", "a white", "a short". It even sounds wrong.
Your example is not at all offensive. It is normal to call black people just "black", white people just "white", etc... even though African or Caucasian is usually used in formal speech, "black", "white" are not at all offensive (but for some reasons "yellow" is considered offensive).