I take the existence of the Guidebook to mean Sana lives in a simulation/game. So when I saw the gun, I thought that Macmosa might be a Dev trying to ensure the simulation is going as planned, or maybe even attempting to stir these simulated peoples conscience. Because it seems that only Sana has a free will at the moment. If everyone had a free will, then everyone would be able to change the future and render the guidebook useless/outdated. Unless, of course, the guidebook updates when the future is changed, in which case everyone very well may have a free will.
Plus, a Dev would be able to spawn in with any weapon, regardless of the time period. So why not a gun, especially if no-one else has one/knows of them. Makes you OP, and seem like a mysterious person with supernatural weaponry.