I gather, from reading the GPG manual, that symmetric encryption based on a password uses one of a variety of Key Derivation Functions (KDF). Although not explicitly mentioned I assume that PBKDF2 is the default as the other options appear to be legacy algorithms from the PGP standard.
However I can't find any information on the number of iterations that the KDF does by default of whether there is a way to change that value manually. I want to know this because it would have an impact on the required complexity for the password (e.g. number of words generated using the diceware method) to achieve a given level of security.