If you have need to migrate Gmail email messages to ProtonMail on Mac & Windows then you can go with the CubexSoft Gmail Backup Tool that is for both Mac & Windows OS. Using it, you can migrate all emails from multiple Gmail accounts to ProtonMail with attachments. The program supports to save selective Gmail emails from large database by multiple filters options. Try it and know how to export email messages from Gmail to ProtonMail without any cost. Read more >> https://www.cubexsoft.com/gmail-backup/