Is GPG symmetric encryption only based on a passphrase? Is the symmetric key derived for AES only dependent on the passphrase?
Is it possible to generate the encryption key such that it is based on my primary GPG private key and temporary passphrase? In this case, the attacker would have to know the passphrase as well as my GPG key to decrypt the text.
In GPG the symmetric key used to encrypt data is generated randomly.  That key is then wrapped (encrypted) using a public key or multiple public keys and anyone with any of those private keys can then unwrap (decrypt) the symmetric key and decrypt the data.  Typically, private keys are protected with passwords.  So yes, under normal circumstances, an attacker would need a copy of your encrypted private key and knowledge of your password in order to decrypt data sent to you.
Notice I said &quot;multiple public keys&quot; above.  So, anyone who's public key was included can decrypt the data so if multiple people are included in then any of them can decrypt the data and the compromise of any of their private keys or passwords would result in that encrypt data being compromised as well.