Hall D is currently using Geant 3
Clas12 is using Geant 4.
Both groups are using HDDS to define geometries. HDDS is written in XML and each group has tools that convert the HDDS geometry into Geant 3 or 4 geometries.
HDDS is supposedly not difficult to learn.
The CLAS12 Geant 4 simulation is not at the level of producing hits.
Geant 4 has been set up to run on the Jlab CUE.
Links
Running Geant4 on the Jlab CUE
Getting Geant 4 for your machine
HDDS Geometry for Geant 3 and 4
http://portal.gluex.org/ Click 'Documents', then 'Public', then type in 'HDDS' in the search box for Hall D HDDS Documentation