Scary Stories to Tell in the Dark has a PG-13 rating. This means it's not for very young kids. There might be some jump scares, creepy imagery, and maybe a little bit of mild gore that could be disturbing for younger audiences.
In a particular FNAF story, a group of kids dared each other to spend the night in the abandoned pizzeria. As the night fell, strange noises started. The animatronics that were left there in a dilapidated state began to activate one by one. The kids realized they were in real danger as the animatronics moved towards them with an unnatural speed. They tried to find a way out but the place was like a maze, and they were constantly being chased by the terrifying animatronics.
I'm not aware of any official connection between GTA 5 and FNAF in terms of scary stories. GTA 5 has its own dark and gritty stories, and FNAF has its own spooky lore, but they are from different franchises and not typically combined in that way.
The back story of FNAF 4 is scary mainly because of the nightmares. The main character is a child who is haunted by these terrifying animatronics in his dreams. These nightmares seem to be related to the dark secrets of Freddy Fazbear's Pizza. The animatronics look more distorted and menacing in this installment, which adds to the horror.
It could be a story where some of the GTA 5's nightclubs or amusement parks have animatronics that come to life at night. These animatronics could start stalking the players, much like in FNAF. They might be found in the dark corners of the city, with glowing eyes, and make strange noises as they approach the player.
In fnaf foxy horror stories, the element of surprise is a big factor. Foxy often appears when least expected, like from the seemingly quiet Pirate Cove. Also, the setting of the abandoned pizzeria adds to the overall spookiness. The dim lights, the creaking floors and the idea that a terrifying animatronic like Foxy is lurking around make it truly frightening.