If compression-use can be specified by the
--compress-algo name option and the default is to 'ZIP'. The default compression level is '6':
--compress-algo n Use compress algorithm n.
Default is 2 which is RFC1950 compression. You may use 1 to use the old zlib version
(RFC1951) which is used by PGP. The default algorithm may give better
results because the window size is not limited to 8K. If this is not
used the OpenPGP behavior is used, i.e. the compression algorithm is
selected from the preferences; note, that this can't be done if you do
not encrypt the data.
You might also want to read http://www.ietf.org/rfc/rfc4880.txt to get an idea of what GPG actually does (point 2.3 and 9.3 describe compression).