GccEmacs refers to the feature/native-comp branch of Emacs, which adds support for compiling EmacsLisp to native code by leveraging libgccjit. All of the Elisp packages shipped with Emacs are native-compiled during the build process, providing a noticeable performance improvement out-of-the-box. Third-party and user modules may also be compiled to achieve further performance improvements.
The official development log can be found here.
The feature is discussed in detail in the talk "Bringing GNU Emacs to native code" here.