After you've installed Xcode (or, my preference, the Command Line Tools via xcode-select -install) so that you can install and use GNAT, you may expect to be able to compile C code too.
Mojave may surprise you with
$ gcc casing.c -o casing casing.c:1:10: fatal error: stdio.h: No such file or directory 1 | #include <stdio.h> | ^~~~~~~~~ compilation terminated.
The reason, according to this question and its answers, is that Apple's developer tools, in particular the clang compiler, know where to find the include files under /Library/Developer; GCC doesn't (I'm sure it could be made to, but ...) and so we have to add an extra step to install them in the normal place:
$ sudo installer -pkg /Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg -target / Password: installer: Package name is macOS_SDK_headers_for_macOS_10.14 installer: Installing at base path / installer: The install was successful.