/usr/include/GNUstep/ProjectCenter is in projectcenter.app 0.6.2-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/include/GNUstep/../../lib/GNUstep/Frameworks/ProjectCenter.framework/Headers
/usr/include/GNUstep/../../lib/GNUstep/Frameworks/ProjectCenter.framework/Headers