An email signature is text, like your contact information or a favorite quote, that’s automatically added at the end of Gmail messages as a footer.
Add or change a signature from Gmail Account (Computer)
You can put up to 10,000 characters in your signature.
Step 1. Login Gmail from computer / laptop
Step 2. In the top right, click on Settings icon
Step 3. Click on See all settings.
Step 4. Scroll page and find Signature Section & Create new Signature
Step 5. After Tap On Create new fill Signature name
Step 6 Enter Your Signature
In the "Signature" section, add your signature text in the box. If you want, you can format your message by adding an image or changing the text style.
Like This
Step 7 Signature Visibility / appearance Setting
Step 8 Scroll and save these setting
Save now.Step 8 Now Compose mail
here you can see signature automatic appeared which added from settingwhen someone receipt your mail
How to add or change signature in Gmail (Android) ?
Add or change a signature
Set up a signature that shows up only for emails you send from the Gmail app.
Open the Gmail app in smartphone.
In the top left, tap Menu.
Scroll to the bottom, then tap Settings.
Choose the Google Account where you want to add a signature.
Tap Mobile Signature.
Enter the text for your signature.
Tap OK.
Now save signature