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
(If you have multiple accounts, pick the correct one)
Go to Signature Settings
Switch Mobile Signature OFF
Please have in mind that signatures in the iOS Gmail app are never visible while composing 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.