From your Gmail account, you can choose to integrate your signature immediately and automatically each time you write a new email.
To do this, go to your Gmail address, then click on the "Gear" pictogram. Then click on "See all settings".
From the "General" tab of the "Settings" menu, you will find the options concerning your signature at the bottom of the page.
Click on the "appended at the end of all outgoing messages" checkbox.
That's it !