According to the docs, the tarfile module can open gzipped tar files directly, so you do not even need to mess around with the gzip module. You could download & save the tar.gz file with ftplib, then do:
(warning: untested code. Caveat Emptor)
Note: the tarfile module was added in Python 2.3, so it will not work if you have an older version.
tar = tarfile.open('path/to/file', 'r:gz')
for member in tar.getmembers():
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