it is the way they seem to be doing it in catia
At the end of the day a CAD system can create basically any BREP shape.
GDML is a set of solid geometries. A far richer set than FreeCAD parts but does not cover all possible shapes.
You cannot have the flexibility of creating arbitrary shapes with sketches and extrude/pocket etc and then have what
you create as GDML solids. McCAD makes an attempt to break down an arbitrary shape to a set of boolean GDML
solids it uses some software called Salome and is complex.
Without access to CATIA I cannot say. Maybe it is like my workbench in that it allows you to edit a GDML file and instantly see the results. Maybe by restricting some CAD functions so it is able to create GDML files, but it cannot offer full flexibility and output GDML those are mutually exclusive.
I am sure you are right in that CERN uses CATIA. It would explain why I don't see clone and fetch requests from Geneva.
On a recent log GitHub sent me just over half request were from China, next largest is US and the rest Germany. Others
viewing pages, but the only clones or fetches were from the three I mention.
Also I think CERN are hoping that the GSOC projects will allow people to use CAD systems to design, Create Tessellated objects from these and then run the analysis in reasonable time using processors with lots of cores.