--symmetric
-c
Encrypt with a symmetric cipher using a passphrase. The default symmetric
cipher used is CAST5, but may be chosen with the ‘--cipher-algo’ option.
This option may be combined with ‘--sign’ (for a signed and symmetrically
encrypted message), ‘--encrypt’ (for a message that may be decrypted via a
secret key or a passphrase), or ‘--sign’ and ‘--encrypt’ together (for a signed
message that may be decrypted via a secret key or a passphrase).

...

--cipher-algo [i]name[/i]
Use name as cipher algorithm. Running the program with the command
‘--version’ yields a list of supported algorithms. If this is not used the cipher
algorithm is selected from the preferences stored with the key. In general,
you do not want to use this option as it allows you to violate the OpenPGP
standard. ‘--personal-cipher-preferences’ is the safe way to accomplish
the same thing.