The Chateu used to belong to Charon. Research notes can be found in Platinum version showing that the Rotom there used to be his (well, never captured it but he played with it). Gardenia is just afraid of the building because of the fact that there are ghosts there. As for the ghosts of the Little Girl and Butler, various reasons could explain why they still reside in the house. However, I believe the most likely explanation is that these ghosts were created by the Gastly, Haunter, and Gengar, likely to scare away people from their home. We have seen this type of behavior from Ghost types before in the Anime. It is unlikely that they were actual people. If they were, their deaths would have to be fairly recent, as Charon and his family resided in the mansion. While possible the Little Girl was some from Eterna City, it is unlikely, as the fact that a little girl died would be talked about among the people. Likely, the Little Girl was Charon's sister, and the Butler was Charon's Butler. The Ghost types would create manifestations of these people due to the fact that they are well known beings to the Ghost types.