I think it's different, because Protonmail can target you, individually, at any time. Like, for example, upon receipt of a NSL.

GPG is used asynchronously. In many cases, everyone would need to be compromised to go after one person. That raises the chance of discovery.

A browser extension is a fantastically better idea. Google previously prototyped most of the work for this: https://github.com/google/end-to-end