from where the centre of gravity is measured
I was unable to open your file. If you could upload in another format (step?) I'm sure it may work better.
In software, the center of gravity is going to be measured from the Origin of the model (coordinate 0.0,0)..
In reality, you'd likely need to define some other point as a reference since the origin could also be difficult to use as a reference unless the model were specifically created with the origin at say a corner of a rectangular part.
If the origin were instead in the middle of the part, or at the center of some oddly shaped face, it would become more difficult to define its position in a real world application (like lifting a large object with a sling).
If you put a point on surface based on components at 0,0, it will always place it at the center of gravity. You can do this if something is rectangular and create an intersection to find the center of gravity. Would be slightly harder if something has different weights at different positions. If it is an awkward shape you can mix both this method to the mathematical method and find a bunch of planes that will intersect to find the CG. Hope this helps! :)
If you have applied material to each of the parts in your assembly, use the Measure Inertia tool to get the COG of thecomplete product.