1. The sounds are the same for fretted and fretless. The GR-20 tracks note, dynamics and pitch bend; and controls a built-in MIDI synth with this information. And the built-in MIDI synth doesn't know anything about the host bass being fretted or fretless.
The sounds are good.
I've recorded a few sound clips and put them here, you may find them helpful?
2. Tracking is good, better in the upper register, increasing delay noticable below low A (roughly). Listen to the two-channel sound clips to get an impression of the tracking.
Note that with any sort of synth-bass sound, your playing probably feels a bit "detached" and artificial. It may sound good, but feel a bit odd. I don't use this for synth-bass sounds. The V-Bass does that better.
3. I'd probably replace it yes, unless there's a new V-Bass coming out soon that has both the traditional V-Bass system
and the GR-20 funcionality built in to the same unit, like the new guitar VG-99 thing.
