OpenPGP is an old encryption standard with almost no use case.

Assuming that every implementation used in an emails thread has no major zero-day, and that the keys are rotated correctly, which is a headache in itself, and way below the current standards (which include double ratchet encryption, and key rotation every time someone sends a new message after someone else). But it's also based on the IMAP and SMTP protocols which can't encrypt whom you're talking with and when. OpenPGP could encrypt the subject but doesn't, which means that even with perfect forward secrecy and no zero-day in any implementation (which would give access to the entire thread) your email provider could read your mailbox, but not its contents.

It does provide some protection against low State surveillance, e.g. against fiscal fraud, in the sense that it doesn't provide a direct proof but it lets investigators narrow down their list of suspects. It doesn't encrypt any data whatsoever and doesn't change anything against mass surveillance. A company like ProtonMailclaiming to fight mass surveillance (or “for a better internet”) with OpenPGP is lying.