How to extract filename.tar.gz file
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 # optionaltar xvf filename.tarOr 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
As far as I can tell, the command is correct, ASSUMING your input file is a valid gzipped tar file. Your output says that it isn't. If you downloaded the file from the internet, you probably didn't get the entire file, try again.
Without more knowledge of the source of your file, nobody here is going to be able to give you a concrete solution, just educated guesses.