For example, the PostScript statement "/Color true def" (written into the file in response to the command set terminal postscript color), may be altered in an obvious way to generate a black-and-white version of a plot. Similarly line colors, text colors, line weights and symbol sizes can also be altered in straight-forward ways. Text (titles and labels) can be edited to correct misspellings or to change fonts. Anything can be repositioned, and of course anything can be added or deleted, but modifications such as these may require deeper knowledge of the PostScript language.
The organization of a PostScript file written by gnuplot is discussed in the text file "ps_file.doc" in the docs/ps subdirectory of the gnuplot source distribution.