Correct me if I'm wrong at any point of this questino.
I understand that AES is a lot cheaper to compute than RSA (or any form of asymmetric encryption) and that that is why GPG and similar programs (the OpenPGP standard?) use RSA to encrypt an AES key in the header to the AES-encrypted body of an encrypted text. This also has the awesome side benefit of being able to encrypt a text with multiple recipients (just encrypt multiple headers with multiple keys; though obviously the security of the message is only as strong as the weakest recipient).
But I have one major confusion about all of this, specifically when I read up on the virtues of 2048 v 4096 bit keys: how secure is the AES encryption of the body? How long is the key? Is it outside of the realm of possibility that the body text itself, independent of the RSA-encrypted key-headers, will be vulnerable in the near-term future? Perhaps sooner than NIST's recommended expiration date for 2048 keys?
I know of no way to specify the symmetric encryption when encrypting with GPG, and believe I have even seen an answer on this board suggesting you cannot specify the symmetric encryption algorithm of the body when asymmetrically encrypting with GPG.