Gretl is written in the C programming language. I have abided as far as possible by the ISO/ANSI C Standard (C89), although the graphical user interface and some other components necessarily make use of platform-specific extensions.
gretl is being developed under Linux. The shared library and command-line client should compile and run on any platform that (a) supports ISO/ANSI C and (b) has the zlib (compression) and libxml (XML manipulation) libraries installed. The homepage for zlib can be found at info-zip.org. Libxml is at xmlsoft.org. If the GNU readline library is found on the host system this will be used for gretcli, providing a much enhanced editable command line. See the readline homepage.
The graphical client program should compile and run on any system that, in addition to the above requirements, offers GTK version 1.2.3 or higher (see gtk.org).
gretl calls gnuplot for graphing. You can find gnuplot at gnuplot.info. As of this writing the current version is 3.7.2.
Some features of gretl (the built-in spreadsheet, the "session" icon window, some file selection dialogs) make use of Adrian Feguin's gtkextra library. You can find gtkextra at gtkextra.sourceforge.net.
A binary version of the program is available for the Microsoft Windows platform (32-bit version, i.e. Windows 95 or higher). This version was cross-compiled under Linux using mingw (the GNU C compiler, gcc, ported for use with win32) and linked against the Microsoft C library, msvcrt.dll. It uses Tor Lillqvist's port of GTK to win32. The (free, open-source) Windows installer program is courtesy of Jordan Russell (jrsoftware.org).
I'm hopeful that some users with coding skills may consider gretl sufficiently interesting to be worth improving and extending. To date I have not attempted to document the libgretl API (other than via the header files you'll find in the lib/src subdirectory of the source package). But I welcome email on this subject and if there's sufficient interest I'll put some time into documentation.