ahhh true, the GR20 is a good unit to produce Piano sounds. its equipt to produce other sounds as well (flute, trumpet, sax, sitar, chinese string instruments, rhodes, organ, etc.) and the sound is "very close" to the real thing!
but u need to place the special pick-up GK3 onto ur guitar for it to pick-up the signal. u have to screw it on the top of the guitar body. or u can double tape it if drilling xtra holes on ur guitar is not an option u want. a few guitar manufacturers have models fitted with the GK pickup (ibanez, fender, godin)
some complaint about the "slow response" and "lagging" in the signal when u play fast. i dun find it noticable the last time i tried it. well, maybe i'm not fast enough! hehehehehheh...