Sorry for the "cocky" response, but if the only reason why you want this particular xbox lightgun is because it looks more cool or realistic, well then I just assumed you were quite young. There is a reason why they don't sell those lightguns in the US. Now for your technical response....
First: yes the PS2 has a USB connector, but that doesn't mean you can just stick any USB device into it and expect it to work. Same holds true for the Xbox.
Second: the GunCon2 is totally different than the Xbox lightgun. The GunCon2 actually has 2 inputs, the USB, and a composite video input plug. The PS2 uses the composite plug to communicate video timing. The Xbox lightgun does not have a composite input. The xbox has many more display modes than the PS2 and as such sends the video timing information over the usb wire protocol. It won't work on anything else but an Xbox.