No.
If you have only Gmail and Protonmail to choose for sensitive stuff it's better to use Gmail + GPG or just Delta Chat with Gmail.

Let me explain you.
The mentioned solutions give you among others encrypted Subject, attachments (haven't tested with DC yet).
Everything encrypted with your private keys stored locally.

On which basis you're so confident that ProtonMail shares your data only  "unless required" because their marketing team said so?