Using Action Replay/Gameshark and a PS2 to cheat on PS1 games...

Let's say for instance that I have a Action Replay and a PS2, can I use it to cheat in Gran Turismo 1 or 2?

No, Action Replay is for Ps2 games only. To cheat on GT1 or GT2, you would need a gameshark lite. The ps1 disc that they give you when you buy the GSv2.2 or AR is defective and doesnt work.