Using Gazebo 7.4.0 and ROS Kinetic, my URDF files that have gazebo extensions like
<gazebo reference="chassis"> <material>Gazebo/DarkGrey</material> </gazebo>
do not convert to SDF 1.6 correctly. This seems to be a bug in the
parser_urdf.cc. This can be recreated by running
gz sdf -p model.urdf and noting that the
material tag is not present.
Perhaps related to osrf/gazebo issue #2103.