[upofadown]

Everything is encrypted with a symmetric key. It is just that sometimes there is an asymmetrically encrypted symmetric key packet included in the message so that GPG (or whatever) does not have to ask you for the symmetric key. This is all fairly generic, if you actually have the symmetric key you can use it directly even if a key packet exists. This means that you can give some entity a key to decrypt a particular message/file without revealing your asymmetric secret key associated with your identity.