According to this:

tracking is touchy at the first few frets of a bass. That’s pretty much a dealbreaker for me when it comes to playing bass because I really like playing in first position. I wouldn’t like the workaround of tuning the patch down, then playing higher up the neck.

I expect guitar won’t have this tracking issue because first position is an octave higher than bass. Still, not being able to use the GM-800 with bass as well as guitar makes it a lot less worthwhile for me.