According to docs you can use
--compress-level etc but does not specify if you omit
-z or other
...-level params if it still compresses.
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).