The octave-gdf package does not use the Octave's pkg mechanism. Its
functions are immediately available upon the installation of the
package in the Debian system, without need of running "pkg load":
$ sudo apt install octave-gdf
$ echo "which gdf_reader" | octave-cli -q
'gdf_reader' is a function from the file
/usr/lib/x86_64-linux-gnu/octave/site/oct/x86_64-pc-linux-gnu/gdf/gdf_reader.mex
This is the reason it does not appear in "pkg list" neither.
So, I think it does not make much sense to create INDEX/DESCRIPTION
files for your octave-zmat package if you are not going down the road
of installing it though the pkg mechanism. This is essentially what
is done for the Octave-Forge packages, which use the functionalities
provided by the dh-octave at build time.