GCC 9 header errors when installed with homebrew

I've got GCC 9 installed via homebrew on macOS Catalina 10.15.1. I'm setting -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk. When compiling a file that includes stdio.h, I get compile errors:

/usr/local/Cellar/gcc/9.2.0_1/lib/gcc/9/gcc/x86_64-apple-darwin19/9.2.0/include-fixed/stdio.h: In function 'perror':
/usr/local/Cellar/gcc/9.2.0_1/lib/gcc/9/gcc/x86_64-apple-darwin19/9.2.0/include-fixed/stdio.h:183:28: error: expected declaration specifiers before '__cold'
  183 | void  perror(const char *) __cold;
      |                            ^~~~~~

/usr/local/Cellar/gcc/9.2.0_1/lib/gcc/9/gcc/x86_64-apple-darwin19/9.2.0/include-fixed/stdio.h:200:1: error: expected declaration specifiers before '__attribute__'
  200 | __deprecated_msg("This function is provided for compatibility reasons only.  Due to security concerns inherent in the design of tmpnam(3), it is highly recommended that you use mkstemp(3) instead.")
      | ^~~~~~~~~~~~~~~~

/usr/local/Cellar/gcc/9.2.0_1/lib/gcc/9/gcc/x86_64-apple-darwin19/9.2.0/include-fixed/stdio.h:219:10: fatal error: _ctermid.h: No such file or directory
  219 | #include <_ctermid.h>
      |          ^~~~~~~~~~~~

Is there any solution to this? Is this a GCC bug or homebrew?