Nothing useable to report yet, but u/beardypig is on the case, worth staying tuned imo
For a PS1 light gun (GunCon or similar) - not with out a custom connection to the GPIO pins or similar. However, a GunCon 2 (for the PS2) is very possible - it’s USB, so easy to connect to a Pi.
I have been working on a kernel module (driver) to support the GunCon 2 that will let you use it as a mouse and joystick which should work well with RetroArch. The GunCon 2 (and GunCon for that matter) work differently to NES Zapper style light guns, they know where on the screen they are pointed at all times (unless the screen is too dark, etc.). The current latest version of RetroArch supports this kind of light gun, as it is essentially an absolute mouse (same as the LCD light gun mentioned - LCD TopGun 3, Aimtrak, etc.).
I plan to release the kernel module soon, but I am still working on testing it. If anyone is familiar with the old drivers for the GunCon 2 (WinGun, etc.) you will know they have an issue with the X axis tracking, I have fixed this issue with the new driver and the tracking is reasonably accurate on a CRT TV (as good as it was on the PS2).