So you want to use a light gun on MiSTer but you’re not sure what you need. Let me help you decide :)
The GunCon2 sync breakout board requires csync from an analog IO board or similar. The other HD15/VGA port can either pass through video or be left disconnected. GunCon2 requires a 15kHz CRT.
RGBs MiSTer.ini settings: composite_sync=1
YPbPr MiSTer.ini settings: ypbpr=1, composite_sync=1