Unfortunately, such option is not available right now. Todoist can be used as an alternative to Google Tasks. We will consider adding such option, though and you're welcome to vote for this request in our Votebox: http://todoist.com/Vote/showProposal/55/
wow, superfast reply. ok thank you very much Brendon.
At least if you're using Tasks in Gmail, a basic workaround is to Email (under the Action menu) a task list (one list at a time), copy/paste the task list from the Email Compose window to a file, and Import the file in Todoist to a project. Only the Task titles transfer, not dates, not notes.
I looked at Todoist because I can't show my tasks sorted by date *and* have indented subtasks - you have to show the tasks in manually sorted order for the indenting to show.
I think as Todoist titles itself as "ToDoist for GMail" then it *must* have some sort of integration with GTasks, otherwise you can't really call it "Todoist for Gmail" can you?
I'm uninstalling, because the title is a lie.