I've tried to use Google's Tasks program to keep track of my action lists, but it has always proven inadequate, at least as far as proper GTD practices go. The main reason I use it and not something else is that it's a Google product, so it works well with my phone. But if it doesn't work well with me, then what's the point? It's not Google Tasks I want, but some program that I can access online or offline, on my desktop or my phone, that allows me to set up sequences of tasks easily and to tag them with contexts so I know what type of action they are and can call up the appropriate list when I need it (such as when I am shopping). Google Tasks is just not doing that, and every other program I've looked at so far seems to omit at least one of those requirements - usually offline desktop access and subtask sequences, and quite often context lists. So if I want this imaginary program, I'll have to write it myself.
Mokalus of Borg
PS - For now, I'm giving Astrid a try.
PPS - It's not bad.