/usr/include/GNUstep/GNUMail is in gnumail.app 1.2.2-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/include/GNUstep/../../lib/GNUstep/Frameworks/GNUMail.framework/Headers
/usr/include/GNUstep/../../lib/GNUstep/Frameworks/GNUMail.framework/Headers