This is how to create a signature in Gmail that is added automatically to new emails and replies. If signatures are set centrally in your organization, this setting may be locked.
Steps
Open Gmail in the browser.
In the top right, click Settings (gear) › See all settings.
In the General tab, scroll to Signature and click Create new.
Enter a name and the signature text. The icons below let you add formatting, links and a logo.
Under Signature defaults, choose the signature for New emails and On reply/forward.
At the very bottom of the page, click Save Changes.
Gmail app
The smartphone app uses its own simple signature: menu (≡) › Settings › select account › Mobile signature.
Tip: Insert a logo via Insert image › Web address (URL) so it's shown to all recipients.