If you type tar --help, at the begining you will see the following:
GNU `tar' saves many files together into a single tape or disk archive, and
can restore individual files from the archive.
Usage: tar [OPTION]... [FILE]...
tar -cf archive.tar foo bar # Create archive.tar from files foo and bar.
tar -tvf archive.tar # List all files in archive.tar verbosely.
tar -xf archive.tar # Extract all files from archive.tar.
Note that there is one (and only one) option to f which specifies the file (device or whatever) to be processed. Note the the [FILE] has a trailing ... That means you can restore(write) many files.
Try the following:
"echo tar -xzf ./*tar.gz"
You will get something like:
tar -xzf ./tarfile1.tar.gz ./tarfile2.tar.gz ./tarfile3.tar.gz
This means you are telling tar to extract the files ./tarfile2.tar.gz ./tarfile3.tar.gz from the file ./tarfile1.tar.gz. This is why it says .the files are "not found in archive."
If you want to do all the tarfiles at once, try the following:
"for file in *.gz; do tar -xzf $file; done"
Of course, you may want to try the following first: so you can see what would happen
"for file in *.gz; do echo tar -xzf $file; done"