Go to
http://root.cern.ch/root/Search.phtml
and look for "g2root" and "gh2root"
An example of another "home-made, but still ROOT-based" solution is present
on:
http://www.star.bnl.gov/STAR/html/comp_l/root/html/St_geom_Maker.html
With my regards,
Valery