Add email signature in Gmail

Add or update existing email signature from a computer

• Open Gmail
• In the top right, click Settings   Settings.
• 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.
• At the bottom of the page, click Save Changes.

Add or update existing email signature from iOS device

• On your phone or tablet, open the Gmail app .
• Tap the Menu .
• Scroll to the bottom, then tap Settings.
• Tap your account.
• Tap Signature settings.
• Switch the "Signature" setting on.
• Add or edit your mobile signature.
• To save, tap Back .

More help can be found at the Google Gmail help page. 

https://support.google.com/mail/answer/8395?co=GENIE.Platform%3DDesktop&hl=en&oco=1