Gmail add-ons that you have installed automatically appear in the Gmail
user interface when you open or compose a message (depending on which interfaces
the add-on provides). Gmail represents each add-on by a
small icon, as specified by the
logoUrl field in the add-on's
Clicking an add-on icon opens the add-on interface. Clicking the icon again hides the interface. The first time you open an add-on, Gmail asks you to authorize it. After authorizing, you can begin to interact with the add-on's interface.
For Gmail add-ons that extend the message UI, the add-on icons appear in a vertical column to the right of the open email thread, under the menu. The name of the add-on appears if you hover over its icon.
For Gmail add-ons that extend the compose UI with compose actions, the add-on icons appear in the horizontal row at the bottom of the compose window. The add-on icons are placed to the right of the standard icons for message formatting, attaching files, and so forth.
On mobile, the add-on icons appear as a horizontal row at the bottom of the currently open message or draft. Clicking an icon opens that interface at the bottom of the message.
For Gmail add-ons that extend the compose UI with compose actions, the add-on icons appear in the compose view's top-right menu.