Responses
According to the novel, Gideon was like Abigail (Baekhap Lee), he somehow knew the plot (maybe because he's originally from the modern world). It's still not formally said on the manhwa, but that's the case. Also, the current chapters' happenings right now are all connected to the later chaps, so it will really create confusion if you missed the other chapters. Feel free to ask any questions!
Did Abigail knew this guy from modern world?
Nope
No wait since when did that dude know about the plot? Why are they suddenly mentioning a plot twist like nothing? Or was it mentioned in earlier chapters?