gnuplot plots 2d and 3d graphs, from a data file or with a formula. It has an interactive mode with online help, or it can be used non-interactively. gnuplot does function fitting to data sets, and it does output to many terminals, among which are PostScript, X11 display, PNG, and GIF (via the old gd library).
gpsd is a daemon that listens to a GPS or Loran receiver and translates the positional data into a simplified format that can be more easily used by other programs, like chart plotters. The package comes with a sample client that plots the location of the currently visible GPS satellites (if available) and a speedometer. It can also use DGPS/ip.
gpsim is a software simulator for Microchip's PIC microcontrollers. It was designed to provide accurate simulation at real-time speeds. Support exists for all three families of PIC's: 12-, 14-, and 16-bit cores. Simulator features like breakpoints (both execution and memory), trace, symbolic debugging, etc. are all supported. Additonally, simulated stimuli like square waves and analog signals are supported or, if you want, you can create your own dynamically loadable modules (like a 7-segment display). gpsim has both a GTK-based GUI and a readline-based CLI.
GPSMan (GPS Manager) is a graphical manager of GPS data that makes possible the preparation, inspection, and edition of GPS data in a friendly environment. It supports communication with both Garmin, Lowrance, and Magellan receivers, and real-time support for any receiver using NMEA-0183. It can also be used in command-line mode.
graphviz is a set of graph drawing tools and libraries. It supports hierarchical and mass-spring drawings; although the tools are scalable, their emphasis is on making very good drawings of reasonably-sized graphs. Package components include batch layout filters and interactive editors for X11, Java, and a TCL/tk extension. The batch filters can be configured as a web visualization service (using GIF and click-maps). Typical applications include display of finite state machines, software diagrams, database schemas, and communication networks.
GRASS (the Geographic Resources Analysis Support System) is a software raster- and vector-based GIS (Geographic Information System), image processing system, graphics production system, and spatial modeling system. It contains many modules for raster data manipulation, vector data manipulation, rendering images on the monitor or paper, multispectral image geocoding and processing, point data management and general data management. It also has tools for interfacing with digitizers, scanners, and the PostgreSQL, DBF, and ODBC connected databases. GRASS operates on all common operating systems.
Gri is an extensible plotting program designed for scientists. It can draw x-y plots, contour plots, and image plots, and has rudimentary programming capabilities. Output is PostScript. Gri is not mouse driven, nor GUI-based; it is a language. Users regard it as an analogue to the LaTeX document formatting language: users gain considerable power, at the price of a moderate learning curve.