Is there a way to encrypt files with AES 256 in the authenticated GCM mode using GPG?
I don't see GCM in supported cipher modes. It seems that the GPG uses only one mode (its own modification of CFB), although libgcrypt supports GCM.
GnuPG relies on Modification Detection Code (MDC) based on SHA-1 for authentication. It's unclear how the authentication performance of the MDC compares with the HMAC in GCM.