I found this on wikipedia:
Many Guncon 2 (PS2) games are compatible with the original Guncon. The only limitation is if the game utilized the extra buttons on the Guncon 2.
* Crisis Zone
* Dino Stalker
* Guncom 2
* Gunfighter II: Revenge of Jesse James
* Endgame
* Ninja Assault
* Resident Evil: Dead Aim
* Resident Evil: Survivor 2 Code: Veronica (EU/JP)
* Starsky & Hutch
* Time Crisis II
* Time Crisis 3
* Vampire Night
* Virtua Cop: Elite Edition (EU/JP)
* Gunvari Collection + Time Crisis (Jap)
But this doesn't tell you what games are compatible with the original guncon, it just says many games are. So does anyone know which games are and which aren't? The only one I found an answer for so far is Ninja Assault, on Gamefly it said the game was NOT compatible with the original guncon.