/usr/include/GNUstep/gnustep/ is in libgnustep-gui-dev 0.20.0-3+b1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/GNUstep/gnustep/gui/
/usr/include/GNUstep/gnustep/gui/GMAppKit.h
/usr/include/GNUstep/gnustep/gui/GMArchiver.h
/usr/include/GNUstep/gnustep/gui/GSAnimator.h
/usr/include/GNUstep/gnustep/gui/GSDisplayServer.h
/usr/include/GNUstep/gnustep/gui/GSDragView.h
/usr/include/GNUstep/gnustep/gui/GSEPSPrintOperation.h
/usr/include/GNUstep/gnustep/gui/GSFontInfo.h
/usr/include/GNUstep/gnustep/gui/GSGormLoading.h
/usr/include/GNUstep/gnustep/gui/GSHbox.h
/usr/include/GNUstep/gnustep/gui/GSHelpAttachment.h
/usr/include/GNUstep/gnustep/gui/GSHelpManagerPanel.h
/usr/include/GNUstep/gnustep/gui/GSHorizontalTypesetter.h
/usr/include/GNUstep/gnustep/gui/GSInfoPanel.h
/usr/include/GNUstep/gnustep/gui/GSInstantiator.h
/usr/include/GNUstep/gnustep/gui/GSLayoutManager.h
/usr/include/GNUstep/gnustep/gui/GSLayoutManager_internal.h
/usr/include/GNUstep/gnustep/gui/GSMemoryPanel.h
/usr/include/GNUstep/gnustep/gui/GSMethodTable.h
/usr/include/GNUstep/gnustep/gui/GSModelLoaderFactory.h
/usr/include/GNUstep/gnustep/gui/GSNibContainer.h
/usr/include/GNUstep/gnustep/gui/GSNibLoading.h
/usr/include/GNUstep/gnustep/gui/GSPDFPrintOperation.h
/usr/include/GNUstep/gnustep/gui/GSPasteboardServer.h
/usr/include/GNUstep/gnustep/gui/GSPrintOperation.h
/usr/include/GNUstep/gnustep/gui/GSPrinting.h
/usr/include/GNUstep/gnustep/gui/GSServicesManager.h
/usr/include/GNUstep/gnustep/gui/GSSoundSink.h
/usr/include/GNUstep/gnustep/gui/GSSoundSource.h
/usr/include/GNUstep/gnustep/gui/GSTable.h
/usr/include/GNUstep/gnustep/gui/GSTextConverter.h
/usr/include/GNUstep/gnustep/gui/GSTheme.h
/usr/include/GNUstep/gnustep/gui/GSTitleView.h
/usr/include/GNUstep/gnustep/gui/GSToolbarView.h
/usr/include/GNUstep/gnustep/gui/GSTrackingRect.h
/usr/include/GNUstep/gnustep/gui/GSTypesetter.h
/usr/include/GNUstep/gnustep/gui/GSVbox.h
/usr/include/GNUstep/gnustep/gui/GSVersion.h
/usr/include/GNUstep/gnustep/gui/GSWindowDecorationView.h
/usr/include/GNUstep/gnustep/gui/GSXibLoading.h
/usr/include/GNUstep/gnustep/gui/IMConnectors.h
/usr/include/GNUstep/gnustep/gui/IMCustomObject.h
/usr/include/GNUstep/gnustep/gui/IMLoading.h