AES Block Cipher Modes

Later FortiOS versions allow you to select GCM for phase2 but you must explicitly select it.  If you don't see those in the drop down for the p2 config, your version is not new enough.  You can also select CHACHA20POLY1305 for the p2.  Now, that being said, I have a feeling you'll only gain from making this selection if you're running on a box with a new enough CPU that has the AES-NI instruction set.  I was about to post a thread asking about this actually, since the FortiOS docs aren't clear.