Separate passwords for different devices
GMail has the option to allocate distinct passwords for separate services and on separate devices e.g. Mail on iPad, selecting this will issue you a strong new password of which that Google service can only be access with on that particular device.

-
Aaron Hartman commented
There is only one gmail feature that really stands out that both Skiff and Proton do not have: application specific passwords. For people who want a managed email service on top of their custom domain, application passwords provide a secure way to have other services use an internal mail account for notifications. Examples are personal and business hosted services, like GitLab, GitHub, Bitwarden, Mattermost, etc.
Being able to create an 'info' or similar account, and create unique passwords per application that uses that email address provides an easy way to manually expire those passwords, and provides limited access compared to the owner of the account (no settings available to applications).