For Mac OS users, there is a system setting that may not allow you to tab onto several types of elements in a web page. To change this setting:
Open Settings on your iPhone or iPad.
Scroll down and tap Mail.
Tap Accounts.
Tap Add Account.
Select Google.
Enter your Gmail email address and tap Next.
Enter your password and tap Next.
If you have 2-Step Verification, follow the prompts.
You’ll see options to sync Mail, Contacts, Calendars, and Notes.
Make sure Mail is toggled on.
Tap Save.
Now open the Mail app, and your Gmail inbox should be there.