There are several symmetric encryption algorithms to choose from, the default is CAST5 (according to this[1] random mail post). This would only be used to encrypt the private key on disk.

Now I'm curious of the methods of decrypting data in transit. Does the NSA have the tools to break PKI based encryption at 1 trillion guesses/sec? I have some wild guesses, but if anyone knows I'd love to hear it.

[1]http://lists.gnupg.org/pipermail/gnupg-users/2010-October/03...