The GP20 has lots of usable amp sounds. They sound not much different to the amp sounds on something like the GT8 to be honest.
The problem for me was there was a distinct lag when switching between manual & memory... not good when you want the overdrive to kick in a specific moment.
GP-20???...personally I think Boss dropped the ball on that one...sounds like complete ass...pretty sure it was only in production for a year or so..at least Boss knew when to cut and run...I believe they'll never get it right(cramming amps into stomp boxes) it's like comparing the taste of real bananas to anything banana flavored... nothing in a stompbox can stand up next to a cranked tube amp...period!....but heres to hoping