Update: I am replying here in case any other users new to gazebo needing to insert a model file from terminal, for whatever reason, can find this thread. I doubt anyone else will suffer the same misconception as I did, but there you go.
I have learned the following things and resolved my core issue, and so will be closing this thread in a minute:
1) I'm a big, dumb, dodo bird
2) gz sdf is not, or can not, be used to spawn an sdf file, but is used to parse, convert, and/or output SDF files from at least the URDF file format, and possibly others??? Maybe it can use the --spawn-file argument to spawn URDF files, but that is not what I needed for the tutorial.
3) The answer to my question was to use the command:
"sudo gz model -f /home/gazebo/.gazebo/models/my_robot/model.sdf -m "My Robot" "
And doing so, I explicitly used sudo thinking it would help guarantee that gz was allowed to read from the .gazebo directory, without any permissions issues arising. This may be a non-issue, but I wanted to be sure.
The model file spawned and was automatically shifted to the appropriate x, y, z, R, P, Y values as defined in the model file.
For future reference, the following manpage explicitly stated what the "gz model" and "gz sdf" commands are for, but I just didn't comprehend it correctly:
http://manpages.ubuntu.com/manpages/b...