ProtonMail Theoretical Security

How does ProtonMail's theoretical security compare with that of selectively using GPG on a normal email account?

Assuming you and the other party both know how to use GPG securely, selective GPG use is almost as secure as ProtonMail (not more). To understand this, consider the data a government agency could compel from your email provider in each case:

ProtonMail:

• Names, timestamps and addresses of all your messages sent and received
• Encrypted text of subject and body of all non-deleted messages

Using GPG with GMail/Hotmail/Fastmail/etc:

• All of the above, plus ...
• IP addresses and access times