find the boundary of a Gizmo plot and clean up the scattered data

I have generated a Gizmo plot, as shown in the attached file (unzip the file then you can see a pxp file). It is in a cylindrical shape and there are 45 degree incline angle for both ends of round bar. This shape was built by a lot of data points. However, there are also some scattered data points outside of the cylindrical shape. My questions are,

1) I want to generate a Gizmo plot to outline the cylindrical shape. How can I find the boundaries and only keep the boundary data points?
2) is there any ways to delete the scattered data outside of cylindrical shape?

Your kind help will be very much appreciated!
question.zip