Google Gmail supports customizable signatures, which it attaches to every email you send. You can add your name, job title, phone number or whatever you want to your signature.
There are two types of Gmail signatures: web signatures for desktop and signatures for mobile devices.
Add a signature to Gmail on the Web
To add a signature, open Gmail and click the "Settings" icon on the top right. It is the small gear icon near your profile picture.
Click "See All Settings" to open the full settings menu.
Scroll to most of the page until you find the "Signature" option and click the "Create New" button.
Name your signature and click on "Create".
Enter anything you want in the box to the right of your new signature. For example your name, the company you work for and maybe your phone number.
Under the word processor where you just added your signature, in the "Signature defaults" section, click on the name of your email signature to add it as the default signature for use in new emails.
In the box to the right of the latter, do the same to use your new signature with the default choice for emails to which you reply or forward.
At the bottom of the page, click "Save Changes" to save your new signature.
You can also add multiple email signatures and switch between them.
Add signature for mobile
You can also add signatures that are specific to mobile devices. These signatures override the desktop versions you just added. In other words, when you send an email from a mobile, Gmail will only use the mobile signature and ignore the signature computer you.
You can use these signatures for anything, but it is common to create one that notifies people that you are communicating via mobile so that they know that you may be slow to respond (if you are out of the office) or that there might be typographical errors.
Of course, you could simply choose to use the desktop signature on all your devices. Do not sign separately for mobile phones.
Click the menu in the upper left corner.
Select "Settings" from the sidebar.
Tap your profile picture or email address to open the signature settings menu.
If you are using an iPhone or iPad, click “Signature Settings” on the next page. On Android, click "Signature".
If you are using iPhone or iPad, change the Mobile Signature button to the right to enable a mobile-only signature.
Add your text and close Gmail to save your new mobile signature.
The custom mobile-only signature will only appear when you use Gmail on a mobile device.
Source of information: howtogeek.com