I can explain why you can't find the .json files for it, it's because this Ghost not a "real" Ghost, just like IT, which is the actual of name of something else in the game I'm not kidding. This "Ghost" is actually an entity like IT where it's hard coded into the game itself inside the data.win file and not an editable .json file. If you ever want to find more secrets and entities of this game, I'd recommend you check out UndertaleModTool because even though it was originally intended for Undertale, it can still open other GameMaker games like GNJ by importing the data.win file.