iOS (iPhone/iPad)
Gmail has discontinued the option to apply the web version automatically (as described below) recently. It should still work for users that set it up before the changes but fails to work if done now. We are currently investigating other options and workarounds and will update this article with more information soon.
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
(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.
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 hurry, re-installing the app will make the signature appear instantly.
Android
The Android Gmail app currently doesn’t support any signatures in HTML.
As a workaround, open the Gmail in your mobile browser and send emails from there. The signature will be appended to your emails.