The Vbass contains a boggling amount of things. Whilst it can be an fx unit with every type of effect under the sun, you can also model amplifiers and cabinets, and then there is the really interesting part.
The pickup interprets the string movement and takes on board everything from scale length to string height to put virtual pickups on the bass so you can play 'other' basses, synths and other things like organs with one bass. I thought it sounded too good to be true but boy was I wrong. Months and months later and after a lot of tweaking and playing, I am confident that the sound is incredibly accurate and the jazz bass for example is the best I have ever personally had, and I get the feel and playability of my #1 bass!!
I know I probably go on about it all the time and bore other people, but it really has opened a whole new world for me! And at the end of the day, the 'clean' bass, or the sound of my ACTUAL bass pickups, sounds better than I have heard so far (I was a pod XT user for a while, tried various other preamps but tended to go direct to desk before that)
I haven't really researched the GR20, not sure what it offers that the vbass does not, but I know a few Vbassers with them, so there must be something. Richard Bona has both and uses them a lot..
Cheers
ped