PostScript font handling is done by the printer or viewing program. Gnuplot can create valid PostScript or encapsulated PostScript (*.eps) even if no fonts at all are installed on your computer. Gnuplot simply refers to the font by name in the output file, and assumes that the printer or viewing program will know how to find or approximate a font by that name.
All PostScript printers or viewers should know about the standard set of Adobe fonts `Times-Roman`, `Helvetica`, `Courier`, and `Symbol`. It is likely that many additional fonts are also available, but the specific set depends on your system or printer configuration. Gnuplot does not know or care about this; the output *.ps or *.eps files that it creates will simply refer to whatever font names you request.
Thus
set term postscript eps font "Times-Roman,12"
will produce output that is suitable for all printers and viewers.
On the other hand
set term postscript eps font "Garamond-Premier-Pro-Italic"
will produce an output file that contains valid PostScript, but since it refers to a specialized font, only some printers or viewers will be able to display the specific font that was requested. Most will substitute a different font.
However, it is possible to embed a specific font in the output file so that all printers will be able to use it. This requires that the a suitable font description file is available on your system. Note that some font files require specific licensing if they are to be embedded in this way. See `postscript fontfile` for more detailed description and examples.