If file filename.tar.gz gives this message: POSIX tar archive,
the archive is a tar, not a GZip archive.
Unpack a tar without the z, it is for gzipped (compressed), only:
mv filename.tar.gz filename.tar # optional
tar xvf filename.tar
Or try a generic Unpacker like unp (https://packages.qa.debian.org/u/unp.html), a script for unpacking a wide variety of archive formats.
determine the file type:
$ file ~/Downloads/filename.tbz2
/User/Name/Downloads/filename.tbz2: bzip2 compressed data, block size = 400k