You can use the Gmail app on the iPhone, you can say that it is in-built on it. It is updated automatically to fix some bugs and technical glitches to make it more smooth to run. If you get that the Gmail app is not updated automatically on your iPhone, then you should confirm that you select the Fetch automatically option on your Gmail account and also confirm internet connection is strong enough. If you still face the same, then contact Gmail by phone. https://www.7qasearch.net/gmail-support-number/