You need to add a gundam icon to the units_32.pcx file in the art/units directory. Also, you need to have a directory in the art/units directory for the unit. When you downloaded the unit it was probably zipped and all you had to do was unzip it right into the art/units directory. After that, go into the editor, go to rules, then units. When you're there, click add unit. Change its stats and icons, and that should do it.