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