Under the search bar on Gmail are a bunch of buttons that allow you to take action on your messages, called action buttons. These buttons appear in the left corner of the Gmail window and can be formatted as per your suiting. All you have to do is click on the Manage Labels option from the left-side menu and select the action buttons you want to appear on your Gmail window. You can also choose to show these action buttons as text. In order to do so, go to Settings > See all settings > Button labels > Texts.