First, you will need to set up your email signature in your web-based version of Gmail. Once it has been set up in your web-based version, your iPhone will automatically use the email signature too. Follow our article on how to add a signature to your Gmail account here.
Wait 4 hours for the web-based Gmail server to sync with your mobile device's Gmail App. Now your email signature will automatically be added to your email messages sent from your iPhone if you are using the Gmail App on iOS.
Note: After you install your signature in the Gmail Desktop app, the Gmail server takes about 4 hours to sync and update the signature across to the Gmail App on your mobile device. Please wait approximately 4 hours to test the email signature is installed correctly by composing an email from your mobile Gmail App. The signature will not show while you are composing an email, it will only show in your Sent items or on the test email when it's received.
Open Gmail app on iPhone.
Then go to the Main Menu.
Select your mail account.
Then open Signature Settings.
Disable a default mobile signature. And that's it!
Note: No need to insert the signature here, as you already installed it on the desktop. It is going to update automatically when you start sending the emails.
If your email signature is not showing when you compose an email, this is normal. You will not see your email signature in the email preview in the Gmail App when creating your email message. To double-check it has been included, you need to send yourself a test email or check Sent items.