You will get that error when you access Gmail, and there isn't really anything you can do about it, except to try different browsers.
This is due to HSTS (HTTP Strict Transport Security) which Google has implemented along with the option to include certificate pinning. This means that in order to access their sites without this warning, you need not only a certificate that your computer believes is valid, but specifically a certificate that Google has said is valid. Without an actual cert that Google has pinned, the connection won't be trusted.
HTST is generally implemented as a header so if you're an active man-in-the-middle you might have a chance to strip the header (or at least modify it to not include cert pins) before it gets to the browser, preventing it from thwarting you, but not even this method will work with Gmail, since Google properties (among others) are pat of the preloaded HSTS set that comes with some browsers, including Chrome and Firefox. This means that your browser comes pre-installed withe the knowledge that Gmail requires HTTPS and specific, pinned certificates.
So, try different browsers, and try to strip any pinned certificates off the Strict-Transport-Security response header before it gets to the browser so that they won't impact your ability to MitM in browsers that don't have Gmail in a preloaded HSTS list.