The other day, a client of mine encountered a nagging problem involving her iPhone that took a little while to resolve. Her iPhone kept rejecting her Gmail user name and password, saying that one or the other was incorrect.
Various forums talked about unlocking a Google captcha or reentering the email account information, but neither of these tips worked.
Turned out the email verification problem was caused by my client having Google’s 2-Step Verification turned on. Deactivating this setting, as suggested on the MacRumors forum, enabled her Gmail account to authenticate properly on her iPhone.