It is very simple actually, as the configuration of a proper machine is already included in the OpenMSX distribution. If you are curious, look for these files: machines/Gradiente_Expert_1.1/hardwareconfig.xml and extensions/DDX_3.0/hardwareconfig.xml.
All that remains to do is to find the correct ROMs and use the correct command line options. Finding the ROMs is quite easy: ask Google. Put the ROMs in ~/.openMSX/share/systemroms/. The final command line should be something like:
$ openmsx -machine Gradiente_Expert_1.1 -ext DDX_3.0