To do app (sync with iPad/iPhone/google calendar)?

No, I think most of these apps sync the tasks via ToodleDo, which you can sync to your Google calendar.

I just downloaded something new I found called GoTasks, which is a free task manager for google tasks. I like a lot more than I thought I would (free usually means feature-poor). It supports subtasking, due dates, and has a nice video showing how it works:

http://shurakov.name/gotasks

There is a separate app for the iphone and for the ipad, but it's free, so not a lot of risk to try.

I had to log in using the "standard" setting on my ipad; the default login method wouldn't work. Not sure what that means in the long run, but it's been working fine. Haven't tried it for long, but so far it's pretty good.

There is an app that does pretty much everything you describe called producteev which initially I found to work very well. Lately I've been having issues with syncing to and from my iPhone though. Still, it might be worth trying it out.

Good luck.