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
Note that when you let a provider do PGP for you, you're not safe from that provider. It's one of the big problems with PGP: you can have usability or security but not both.