The easiest way to import emails from a different server I can think of is the following:
- Install Thunderbird on your computer
- Setup Thunderbird, so that it can access the gmail account via IMAP https://support.mozilla.org/en-US/kb/thunderbird-and-gmail
- Setup Thunderbird, so that it can access the ecloud-mailservice via IMAP [HOWTO] Email Configurations - to configure /e/ on most email clients
- Use drag and drop to move the emails from the gmail account to the ecloud account.
- (optional) remove the gmail account from Thunderbird