There's no board for the GR-D/GR-S and I got the 'old thread' warning when I went to post at the one place they seem to be discussed on this site, so I'm starting a new one. I wasn't sure what my purpose was for writing this, but it kind of sounds like a review, so I'm putting it here.
At the moment, I'm buying any (sub-$200 or so) 13 pin stuff I come across, and I happened to find a GR-D at a local store a week or two ago. I had a mild interest in the GR-S, so I was kind of disappointed to find its brother, but I went for it anyway. It set me back $80 which might be too much...I don't know.
I've never been a big distortion user and in fact, this is the first dedicated distortion pedal I've ever owned. I have to say though...I like it. The sounds are really synthetic, which is a good thing for me. It sounds like more of a synth-overdrive effect than a distortion pedal, which makes it more useful for my purposes.
All of the sounds it can create are available on the GP-10 for the most part. However, the 2nd distortion sound doubles straight distortion with a wave synth, which you can't strictly do on the GP-10 (to my knowledge). You can have one or the other, but not both at the same time.
At high gain settings it's a bit too loud, but I think that's due to my piezo pickup rather than the pedal itself, since I've seen the GR-D described as 'no noise' distortion.
Anyway, I don't know how much use I'm going to get out of it, and I definitely don't think it is worth $300 or whatever they were charging when it was first released, but it's a fun thing to mess around with. If it weren't for the noise, I might even try to incorporate it into my arsenal somehow.
Does else anyone care about these things at this point or are they too one-dimensional/redundant?