/usr/include/GNUstep/gnustep/gui/GSTextConverter.h is in libgnustep-gui-dev 0.22.0-1ubuntu2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | #warning GSTextConverter.h is now included using the path <GNUstepGUI/GSTextConverter.h>
#include <GNUstepGUI/GSTextConverter.h>
|