How can I implement a GTD system in Google Tasks in combination with GMail and GCal?
Google Tasks is a relatively basic system compared to other task management systems, so you're not going to get all the features (tags, locations, priorities, etc.) of other systems (like Remember The Milk). But it will definitely work, as it supports lists and moving tasks between lists.
First, set up several lists according to GTD (inbox, next actions, someday/maybe, waiting, etc.). You might want to set up separate "Next Actions" lists per context, since Google Tasks doesn't support tags. Next, collect into your Tasks inbox and your Gmail inbox. Finally, process out of those two inboxes into your lists system (the goal should be to keep an empty Gmail inbox and an empty Tasks inbox). Any actionable emails can be turned into tasks using Shift+t. Any day/time-specific actions can be put in Google Calendar (which fits the GTD recommendation of having both time-specific entries as well as "all day" entries). The tickler file can be implemented using reminders on the calendar.
The problem I've got with using GTasks for GTD is you can't organize by both project and context (among other issues). Personally I don't think you can develop an effective GTD implementation using GTasks.
If you just need the basics, I'd set up your lists by context. Use calendar for ticklers and appointments and mail for collection. But, as @jrdioko points out, you'll need multiple lists for someday and next-actions.
I think it's a round peg in a square hole.