If you have an existing GTD system, look for an Alexa skill that'll feed into it. But, I've tried this with Reminders/OmniFocus, and finally gave up because it was too unreliable (I think it required a connection through IFTTT or something, and some piece would break, leaving my items in Alexa's todo list and not in Reminders). If you find an Alexa skill specifically for the list manager you use, you might get a more reliable setup. You could also, in theory, use the Alexa app's built-in lists feature, but that wouldn't be a very robust GTD system.