When used in a Perl context, GD offers a Perl module with the gd graphics library. The reason for saying "the Perl context" is because the gd graphics library was originally developed for C rather than Perl, and in fact, when downloading the GD Perl module, the latest version of gd is also required, since the GD module also includes C code, which requires gd for error-free compilation.
The GD Perl module supports the JPEG and PNG graphics formats. Previously, the GIF graphics format was supported, but due to action by a company called Unisys, GIF was dropped and the code was rewritten to use the PNG format instead.
GD is portable across Unix operating system variants such as Sun Solaris and Linux.