A great plot could be that Glamrock Freddy and Gregory find an old animatronic from a previous iteration of the Pizzaplex that holds the key to an ancient mystery. They have to work together to figure out what the mystery is and how to use the key, all while avoiding the other animatronics who are acting strangely.
Develop a backstory for their relationship. Maybe Gregory reminds Freddy of a child he used to interact with in the past. Or perhaps Freddy was the first friendly face Gregory saw in the Pizzaplex. By exploring their pasts and how they came to meet, it makes their relationship more believable. For instance, you could write about how Gregory was initially scared of Freddy but as they faced challenges together, they grew closer. Freddy could start to see Gregory as more than just a lost child, but as a friend he needs to protect at all costs. This kind of development over time makes their relationship feel real and engaging for the readers.
I'm not sure specifically who 'Gregory Green' is in this story without more context. It could be an ordinary person with his own unique experiences, or perhaps a character created for a particular fictional narrative.
Yes, he has. Markiplier often does unique and entertaining segments, and this could be one of them. He likes to interact with different types of content related to his fan base, and reading fanfics, even the not - so - good ones, is a way to do that.
I don't think it's a true story. Such heist scenarios are often the creation of writers and storytellers to provide thrilling and adventurous experiences. There's no evidence to suggest it's based on reality.