You just answered your own question. Yes. You still use GTD even if you have a perfect memory, because GTD is based on the natural planning process.
Originally Posted by supergtdman
What you're describing isn't not having a GTD system. It's having a very streamlined one. And I guarantee you that to get things into that system, you will still decide that they belong in the system (Process) and then later look at them to decide that they're the next relevant thing for you to do (review). Those steps might take so little time and mental energy that you don't even notice them happening, but they're always there.
The problem with memory is that even if you can trust it to remind you of every single thing you need to do to make the most of your time in context A, it will probably still bug you about context A when you're in context C. That's a waste of your energy and a source of undue stress. So why not externalize it, so that you can be fully present to the task at hand?
There's an apocryphal story that Einstein never memorized anything he could look up. I think that's a bit extreme, but it does illustrate the point--the mind is a great place for having ideas. Not a great place for storing them. Free up the storage, and you have more room to work.
Collect. Process. Organize. Review. Do. That really is all there is to it.