Build gccemacs for macOS 10.15 or newer using Nix.
Build on local machine
git clone https://github.com/twlz0ne/nix-gccemacs-darwin
cd nix-gccemacs-darwin && nix-env -iA emacsGccDarwin
Pull pre-built binaries from Cachix (recommend)
nix-env -iA cachix -f https://cachix.org/api/v1/install
cachix use gccemacs-darwin
nix-env -iA emacsGccDarwin -f https://github.com/twlz0ne/nix-gccemacs-darwin/archive/master.zip
For macOS 10.12 see nix-gccemacs-sierra.