Google encourages you to set a second email address on your gmail account so that if you forget your password it can be reset by sending a link to your alternate email address. Your recovery email is also used to inform you of suspicious activity on your account. However, you cannot do this from the tablet app – i.e. the Gmail app.
You have to login in to your gmail account through a web browser. When you have logged in, click on your profile icon or picture and choose account. Here you can scroll down to your recovery email. Click to change.