Comment by giuliomagnifico

As far as I know, Gmail doesn’t allow saving PGP keys or using them to write new emails, whereas mailbox.org integrates the entire PGP service and to send an email, even from iOS where PGP apps are "ugly," you just need to do it from the web interface.

Anyway I wrote the details in the post.

Edit: I have to mention that I generated my PGP keys locally and then imported to Mailbox.Org