I believe with the GPZ the motion speed is automatically assigned depending on your settings i.e. high yield, general, extra deep.
Much like using the pre sets on the GPX4500/5000 but not adjustable.
Target volume is much the same, assigned by settings I believe & there is some adjustment via threshold volume + audio smoothing.
The GPX6000 I believe does similar based on sensitivity setting/auto & feedback from Geosense but doesn't have any threshold adjustment as such. The sensitivity settings (IMO only) acts very similar to an audio smoothing type setting & really doesn't overly affect sensitivity as such greatly. Even on low settings it's still very sensitive.
The coils attached may also feed into this? Not sure.
Got a chance to use the 17" GPX6000 coil in some ground I knew had deep goodies. Previously had thought there wasn't too much depth gain with this coil but was pleasantly surprised (& a bit sore after digging a few deep holes). Makes me think that the GPX6000 may also adjust things to suit the coil attached? Unfortunately not a lot of info from Minelab on the 6000 or Geosense other than basics.
BC needs to give us a knowledge based article with more in-depth info on how it all works.