How to tell Xcode13 to use GCC for building C++ projects?

I have GCC installed and require the ability to use some libraries that are only supported in C++20 for now. In the project build settings in Xcode, how do I use the GCC compiler instead of the libc++ compiler used by default?

This question appears to have been asked earlier last year however has not been answered.

GCC is no longer supported by Xcode.