The casino is based on the Ashbury Park Casino in Ashbury Park, New Jersey, which is also abandoned, but is currently undergoing renovations.
The Alderney Casino can be entered and explored freely. It has a very large interior, but it is mostly filled with rubble. The whole building appears to be very old and in a state of disrepair. Large holes are present in the ceiling, and there are no doors or windows. Many hobos spawn inside of the building.
The casino is the location of certain myths. Players and users have reported that they have heard voices or Growling Noises inside the building. There have been many videos online apparently showing proof of these voices inside the casino, but some of them could be mistaken with hobos. Also, the player may hear strange knocking or creaking noises inside the casino. This is an ambiance noise that was programmed to play inside the casino, and it may just be the sounds of the old building settling and shifting.
In a mission in GTA IV, the player has to hunt down and kill Jimmy Pegorino inside the casino. After this mission, players have reported seeing his ghost in the building. It may be cause of small sources of light.
Nevertheless, the casino is definitely a spooky place to explore at night, and it remains unknown why it was abandoned in the first place.