to commonly asked questions.

Why does this gmail message appear?

Why do I keep getting this message constantly?

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Google (gmail) has added additional layers of security to their email service. Easiest thing to do to resolve this would be to delete the account from the device and re-add it once again. However this time when adding the gmail account, you'll see a slightly different looking window that opens (Safari) that will give more thorough access to the iPhone. If it continues then there is a settings that be found directly in the gmail settings at that would allow the account to be accessed on what Google considers "less secure devices". You can turn that on and that to, may resolve the issue but mostly likely, just deleting and re-adding the account should resolve it.
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