Coin3d is used for the rendering bit AFAIK. I think Zolko talks about improving the measurement tool within FreeCAD.
[Geometric Constrain Solving](https://en.wikipedia.org/wiki/Geometric_constraint_solving) is about finding a solution when you define constraints between geometric elements, such as A tangent to B or C perpendicular to D. If you use FreeCAD's sketcher, you get a good feel about what GCS is. [Solvespace](https://solvespace.com/index.pl) is another parametric CAD system. Surprisingly good to be honest. Their GCS is provided as a separated library [here](https://solvespace.com/library.pl). I played with it too, but I don't remember how far I got (see [solvespace.nim](https://github.com/mantielero/solvespace.nim)).
