Instead of adding an extra app, many people prefer to use Apple’s built-in email app, Mail. If you want to add your Gmail address to Mail follow these steps:
Go to Settings > Passwords & Accounts.
Tap Add Account, then select your email provider.
Enter your email address and password.
Tap Next and wait for Mail to verify your account.
Choose information from your email account, like Contacts or Calendars.
For more steps visit Gmail configuration on iPhone.