/usr/include/gpac/internal/ is in libgpac-dev 0.5.2-426-gc5ad4e4+dfsg5-3.
This file is owned by root:root, with mode 0o755.
..
/usr/include/gpac/internal/avilib.h
/usr/include/gpac/internal/bifs_dev.h
/usr/include/gpac/internal/bifs_tables.h
/usr/include/gpac/internal/camera.h
/usr/include/gpac/internal/compositor_dev.h
/usr/include/gpac/internal/crypt_dev.h
/usr/include/gpac/internal/dvb_mpe_dev.h
/usr/include/gpac/internal/ietf_dev.h
/usr/include/gpac/internal/isomedia_dev.h
/usr/include/gpac/internal/laser_dev.h
/usr/include/gpac/internal/m3u8.h
/usr/include/gpac/internal/media_dev.h
/usr/include/gpac/internal/mesh.h
/usr/include/gpac/internal/mpd.h
/usr/include/gpac/internal/odf_dev.h
/usr/include/gpac/internal/odf_parse_common.h
/usr/include/gpac/internal/ogg.h
/usr/include/gpac/internal/reedsolomon.h
/usr/include/gpac/internal/scenegraph_dev.h
/usr/include/gpac/internal/smjs_api.h
/usr/include/gpac/internal/swf_dev.h
/usr/include/gpac/internal/terminal_dev.h
/usr/include/gpac/internal/vobsub.h