For now, Google Tasks Indicator supports adding new tasks, marking tasks as completed (or unmark them) and clearing completed tasks. This is the first version, released for testing purposes, so there are still things not yet ready, like editing tasks, selecting the list, configuring the number of tasks displayed and so on, but these features should be available soon. Also, for now Google limits the Google Tasks Indicator API to 5000 hits per day, but hopefully this limit will be increased.
Install Google Tasks Indicator
sudo add-apt-repository ppa:atareao/atareaosudo apt-get updatesudo apt-get install google-tasks-indicator python-gflags
sudo apt-get install python-gflags