For your email signature to work in the Gmail iOS app, please first refer to one of our Gmail browser installation guides and set up your email signature in your Gmail account settings from your browser.
When your email signature is set up within your Gmail account settings, open your Gmail iOS app and tap the hamburger icon at the top left of your screen (three horizontal lines), then scroll to the bottom and tap Settings. Select Signature Settings, then disable Mobile Signature.
Once your Mobile Signature is disabled, you can compose a new email and the app will pull in the email signature you have installed in Gmail from web.
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Gmail Email Signatures