Despite gnuplot's name, it is not part of or related to the GNU

GNU

GNU is a Unix-like computer operating system developed by the GNU project, ultimately aiming to be a "complete Unix-compatible software system"...

 system (hence the decision to use a lowercase 'g') and it is not distributed under the GNU General Public License

GNU General Public License

The GNU General Public License is the most widely used free software license, originally written by Richard Stallman for the GNU Project....

 (GPL).
However, some GNU packages do use gnuplot.

The title of this program was originally chosen to avoid conflicts with a program called “newplot”, and was originally a compromise between “llamaplot” and “nplot”.

The program is distributed under a license which permits copying and modification of the source code.
However, modified versions are only allowed to be distributed as patch

Patch (computing)

A patch is a piece of software designed to fix problems with, or update a computer program or its supporting data. This includes fixing security vulnerabilities and other bugs, and improving the usability or performance...

 files.
For script-driven graphics, gnuplot is by far the most popular program.