Drumroll please, iOS Gmail now supports media queries/responsive design.
Not sure if I'm late to the game here, but I just downloaded the latest update of Gmail on my iPhone and to my surprise, Media query support! Fully responsive!
Go to your appstore and update and see all your emails in their finest glory, and of course, see any possible issues that may exist.
One note is that you should not use attribute selectors in your CSS, as Gmail does not yet support these (at least last time I checked.)