I am in the process of developing a GDML workbench see https://github.com/KeithSloan/FreeCAD_Python_GDML
My understanding is that CERN and Jefferson in the USA need to run the designs of their machines through Monte Carlo simulations.
The software for this is ROOT and Geant4, which can take as input a GDML file.
There are challenges taking designs in BRep format ( i.e. STEP) files and performing Monte Carlo simulations on them for reasons I don't really understand, after all FEM uses Mesh etc. The problem is that the simulations just take way too long to run.
There have been attempts to convert to Tessellated GDML file and also an application called McCAD that tries to break down BRep objects into CSG type soiids.
My workbench implements GDML solids ( Much richer set than FreeCAD csg) as FreeCAD Python Objects and as a result one can change
the values by editing the properties which are the same as defined by the GDML specification/manual.
Anyway to date my workbench repository has received 4 stars and 2 thumbs up in the Geant4 forum. but none of these people work for CERN.
One of the test files in my repository is lhcbvelo.gdml which I assume came from CERN. I would be very interested to test with others.
I am under the impression that CATIA has some facilities for creating GDML, but as CATIA is very expensive and not open source, I don't know much more about it. SolidWorks may also have similar facilities.