If you download the GVP install/tools disk (which is/was available at
http://www.gvp-m.com/), you can use the GvpCPUCtrl command to go into 68000 mode:
GvpCpuCtrl BOOT68000
It's quite simple to modify your startup-sequence so that command is called if e.g. the left mouse button is pressed while booting.
-- M