A few days ago, Google announced Google Keep. (drive.google.com/keep/
It's an app/service like Evernote, where you have the possibility to keep notes and tasks. It have much in common with Gmail Tasks.
Unfortunate, there is no sync or integration possible yet between Google Keep and Gmail Tasks.
On this moment, Rainlendar only has the possibility to sync with Gmail Tasks. I really think, it would be great to integrate Rainlendar with Google Keep. I think, Google Keep is the successor of Gmail Tasks. It would not surprise me if they are going to discontinue Gmail Tasks in a few months.