Good news, Dreamcast lightgun games work on a VGA monitor (using the DC-VGA box). Bad news, DC lightgun games will not work through the s-video/composite cable run through to a vid capture card on your computer monitor.
Good news, Playstation1 guncon games will work on a TV. Bad news, Playstation 1 guns/games won't work on a computer monitor no matter what you try.
Good news, Playstation 2 (guncon2) games will work on a computer monitor if you upscan the signal (I have only tried with the xrgb-2 upscanner and it works). Bad news, Playstation2 guncon2 games will not work through the s-video/composite cable run through to a vid capture card on your computer.