Users face login issues when they can't log in to Gmail through the service's login page. So, if you can't sign in Gmail on iPhone, check the server status and see whether Google is down in your area. If there's no server issue, then the problem is with Google. You need to wait. As soon as the server is up, you'll automatically access your Google account. Further, this issue can also occur due to inaccurate incoming and outgoing server addresses. Therefore, you should check your server address and authentication settings and also verify that you've entered the correct email address and password.
Read also related to gmail issues –
Why is my gmail not loading new emails
Gmail Won’t Send Emails Android
How to Resolve Gmail not Receiving Emails on Laptop
create Gmail Account for business