Simply:
Download the GDEM zipped files (see here on how to do that).
Unzip the zip files in the working directory.
The working directory should contain sub-directories containing the GeoTIFF files.
Run the script giving -h for help or the final DEM name as arguments.
The Final DEM is aimed for using as DEM in ROI-PAC interferometry processing.
If a -g is added as an argument, the produced DEM will be suitable for interferometric processing using Gamma software.