PostScript doesn’t support transparency but has extensions that allow transparency when the ps file is converted to PDF. The use of it has changed very recently when, for security reasons, ghostscript 9.25 dropped the -dSAFE keyword that was used by GMT and other software for transparency in PDF.
Another useful ps extension is the one that allow embedding geographical info and produce the so called geoPDFs that show coordinates on mouse movements when displayed on AcrobatReader. GMT produces such geopdfs.