From the German Techradar page, some mid and high end gaming monitors have an extra feature, which can turn on a gun sight for those games that don’t have their own. Monitors, such as the Gigabyte Aorus FI27Q-X have an option in the on-screen menu to turn on a gunsight overlay for games that don’t have their own, like Rust.
I’m assuming that it simply overlays the cross-hairs in the middle of the screen, to make targeting easier.
https://global.techradar.com/de-de/news/dein-pc-monitor-konnte-eine-heimtuckische-funktion-haben-die-dich-bei-spielen-besser-macht