It is Naked Browser telling you that. If you turn off the "Security warnings" option in Naked Browser settings then it will not show the security messages. It will still turn the URL bar from yellow to white, indicating some security issue.
Usually, there really is nothing to worry about but it is letting you know that there exists the potential for someone to monitor or even change the data to/from the browser.
Finally, I notice that the security issue only exists on the gmail mobile site. If you switch to the "desktop" user agent then there is no error.