The option “leave message on server” is, as Daniele says, a POP3 config setting rather than IMAP.
The messages will have been removed from hispeed.ch.
You can forward them all back if you wish, from gmail. If gmail has a “redirect” option then use that instead of “forward” for your emails, as that preserves the sender info which otherwise becomes your gmail email address (although the original sender will show in the message itself)..