This is quite annoying for who cares about security.
- create a backup copy of gnotifier.exe (usually in c:\Program Files\Google\Gmail Notifier);
- open gnotifier.exe with an hex editor (emacs hexl-find-file will do);
- find the string http://mail.google.com/mail and substitute it with https://mail.google.com/mail;
- save the file;
- close any running GMail Notifier and launch it again.
yes, it is as simple as that.