/usr/share/GNUstep/PostScript/GSProlog.ps is in gnustep-gui-common 0.26.2-3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 | %%LanguageLevel defines
%%
%% Copyright (C) 2005, Free Software Foundation, Inc.
%%
/_GSsetalpha { pop } bind def
/setalpha where
{pop /GSsetalpha /setalpha load def}
{/GSsetalpha /_GSsetalpha load def}
ifelse
|