This extension is available on GNOME Shell extensions website with the name "GTG Integration". To install it, you can simply go to the website, search for the extension, change the switch button to "ON", and accept download. It is now installed. This extension bring many features described in next section.
This feature is simple to use. Open the overview by pressing "super" key or move your mouse on top left corner or click on "Activities" button, type some letters to search in your tasks, and the extension will displays results. The task editor will be open if you click on a task.
The second feature bring with the extension is the calendar menu which can be open by clicking on the date in the topbar. For now, the GTG calendar menu will replace the existing, synchornization of both is planned for the future. This menu can be split in two others, tasks menu and todo list.
This menu shows GTG's tasks to do for selected day in calendar menu, on the left. You can see two different colors for your tasks. The first one (Grey) displays tasks which has start date identic to the day selected and the second one (Dark grey) displays only tasks with a start date before the day selected and/or a due date after the day selected.
On the right side of the calendar menu is the todo list. This menu displays only tasks without start date AND without due date. By this way, every tasks are listed to user.