So what many people refer to as GNU C is actually part of a collection of software development tools. The key value of the GCC project is that it has resulted in a compiler and software development tools that provide a particularly high level of code portability across OS platforms. That is of interest to software developers who want to produce binary (executable) software distribution that will run across many hardware and OS platforms with minimum code change. GCC delivers well in this regard.
The GNU home page describes the scope of the GCC project as:
"GCC development is a part of the GNU Project, aiming to improve the compiler used in the GNU system including the GNU/Linux variant. The GCC development effort uses an open development environment and supports many other platforms in order to foster a world-class optimizing compiler, to attract a larger team of developers, to ensure that GCC and the GNU system work on multiple architectures and diverse environments, and to more thoroughly test and extend the features of GCC."
This was first published in June 2003