1. Send yourself an email from Gmail that already has the signature set up. It can be a blank email as long as it has your signature.
2. Launch the Mail app from your Home screen and open the email you just sent.
3. Tap and hold the signature so that the selection tool appears.
4. Tap Select All to highlight the entire signature.
5. Tap Copy.
   
6. Go back to your Home screen.
7. Launch Settings from your Home screen.
8. Tap Mail.
9. Tap Signature under the Mail section.
10. Tap and hold the default signature so that the selection tool appears.
11. Tap Select All to highlight the entire signature.
12. Tap Paste to insert the signature you copied from the email.
13. Shake your iPhone or iPad to activate the Undo tool. (It's possible you have "Shake to Undo" function deactivated. To switch it on, go to Settings->General->Accessibility->Shake to Undo and toggle it on).
14. Tap Undo to remove any automatic formatting and bring back color or content from the original paste.
    

Your signature is ready and will now appear at the bottom of emails sent from your iPhone or iPad.