When you're on your Android device and you send an email from the Gmail application, you might want your recipient to know that your email has been sent from your smartphone. For this, there is the custom signature, apart from having a signature for show. It would be a nice idea to inform the recipient that you are working at your iPhone.
Here's how to add your personal signature on your smartphone Android
1- Start the Gmail application
2- Press the menu button
3- Select "More"
4- Select "Settings"
5- Choose "Signature"
6- Enter your text
The text entered will become your signature.