I have
a GR33, and after using
the GK2A on various guitars, ended up getting a MiM Roland ready Strat.
First off, you either need two sided tape or be prepared to drill two small holes in a guitar. I even drilled in an inexpensive guitar, and still did not like the installation. Also, the added on pickup looks and feels clunky. There is a chance that your current guitar case will not fit, if it is a tight case.
Additionally, the pickup NEEDS to be RIGHT by the bridge, and about a credit card thickness away from the strings. You should then be able to make final sensitivity adjustments on
the GR20...as I can on the GR33.
After having the Roland ready Strat for a good while now, I realize that it is actually far superior, and the controls are getting easy to jump to. Having a separate guitar volume AND a synth volume is a big plus for me, as I run a separate amp for the synth stuff in addition to my guitar amps. If getting a used one off Ebay or a new one isn't appealing, you might also consider just getting a cheapie Squire Strat or similar, as I think the pickup would fit best on that style guitar. Take your pickup if looking for something along that line, as string spacing IS critical and does vary as you know.
I know you have the pickup already in route, so I would just use the two sided tape as a starter, then get comfortable with the synth in general. If you need to get serious... ala playing out live with it as an example, I think you'll wanna get a factory type installed setup, rather than an "add-on" type pickup.
btw..congrats. I completely LOVE the Roland setup. I have very minor complaints at this point..