Google’s Gmail application for iOS does not perform what’s known as “certificate pinning” This means an attacker can view plaintext emails and steal credentials in MitM attack. The vulnerability does not affect Gmail applications running on Google’s Android or any other Android operating system. Google has not yet responded to the issue, but the vulnerability remains exploitable in Gmail’s iOS app at the time of writing. Google validated the problem and told the company it was fixed, but still the security vulnerability still exists.
Source: https://thehackernews.com/2014/07/gmail-app-for-ios-leaves-users.html