iOS (iPhone/iPad)
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
Make sure you're logged into the correct account
Go to Menu
Scroll down to Settings
Go to Signature Settings
Switch Mobile Signature OFF
And here is how it will look while composing a new message:
When replying to/forwarding an email in the app, the signature will be added to the very bottom of the email, nevermind your signature position settings in Gmail.
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're in a hurry, re-installing the app will make the signature appear instantly.