8-bit console light gun trouble
The gun is AC-coupled, and it is probably looking specifically for the 60-Hz pulsations that would come from an NTSC CRT monitor/TV set. It would be tuned for this in order to do a better job of rejecting unwanted signals from other sources of light, including DC (sun, incandescent bulbs) and 120 Hz (fluorescent bulbs).

Note that the light from an LCD monitor (the backlight) will either be DC (LEDs) or a very high frequency (CCFL).