The signature in Gmail for iPhone/iPad doesn't have to be set specifically, the same design as for desktop will be used. If the signature doesn’t appear to the recipient, make sure you don’t have active a specific mobile signature that rewrites your default signature. To do that:
- Open Gmail app on your iPad/iPhone
- Go to Menu
- Tap Settings
- Switch Mobile Signature “OFF”
- Save your changes
Please have in mind that signatures in the iOS Gmail app are never visible while writing an e-mail but added to the message after sending. The signature might take up to an hour after the above described procedure to appear by the e-mails sent from the app. If you hurry, re-installing the app will make the signature appear instantly.
The Android Gmail app currently doesn’t support any signatures in HTML. Support for signatures is currently in development.
As a workaround, open the Gmail in your mobile browser and send emails from there. The signature will be appended to your emails.