To address your concerns, every major browser supports css media queries. Gmail could simply hide the elements when the user tries to use the browser's print button. Again, this is designed to remove plausible deniability, not to provide an actually secure service. It's still email after all.
Very nice catch -- I checked and they do use media queries during print-to-pdf.
Saving the webpage itself works fine though -- which again, is a browser feature your secretary probably knows about and is comfortable using.
I guess my objection to the idea that this is just removing deniability is that it really doesn't feel to me like it's being marketed that way. I wouldn't call a service like this "confidential mode", I would call it something like "auto-delete mode". Maybe I'm just arguing over semantics though.
