As for the GT3, It's great. But if your willing to through in an extra $200 I suggest that you purchase a GT5. My friend has a GT5 and it comes with alot more features. The only problems I've encountered with the GT3 are (1) the wah pedal is hard. It doesn't give as much control of wah as the crybaby. I was looking for a Satriani type of effect but just didn't get it. (2) The manual isn't all that great. I had to find out for myself how to adjust settings. (3) If you don't have the right equipment to go with the pedal you get unwanted sound from your effect. I use the pedal with one Fender stage 112 amp. At low volume the sound is great but turn that baby up and you get a whole lot of feedback. My friend though uses rack equipment and cab speakers with his GT5 and he creates perfect sounds. Just keep in mind of these things.
I never heard or seen what a GT6 can do, but when I look at what it's going for it's just a little bit better than a GT3. Better to purchase the GT5.