A question I get asked very often when people realize I use the Getting Things Done system to organize myself is
And the magical answer you are waiting for is..... It doesn't matter. The importance is that you have a tool where you can offload it from your psyche.
Your tools can be as simple as paper, pen, folder and in baskets all the way to super modern hi-tech tools like The Brain. Experienced practitioners will usually choose a specific tool for each of the main GTD functions.
As an example, in the GTD methodology, you need some kind of filling system for Reference. Some people, even David Allen, use a traditional filling cabinet and folders. Over the years I have tried different systems and have settled on using Evernote for my Reference system.
This doesn't mean Evernote is necessarily the best tool for the Reference system in GTD but it is the easiest and best for me. Its the one I chose after testing various systems. Remember that your tool shouldn't be more complicated than the function you are trying to perform.
Evernote is also my capture and Tickler system.
Don't forget that the core of GTD is lists (such as Next Actions, project, Some Day, Waiting for, etc). Lists are a simply thing and can be done and easily maintained on paper, on your smartphone's free note taking app, in Evernote, Google Keep, Workflowy, etc. The success of a GTD implementation is not the tool but rather the process.
Some newcomers to GTD falsely believe that getting a "GTD purpose built tool" will make them better GTDers fast. This is false. If you never drove a stick shift and I gave you a Ferrari, chance are you wouldn't get very far.
The worst thing you can do it try to learn GTD at the same time as you are trying to learn a new tool. This is a recipe for disaster. Once you understand the different requirements of GTD, ask yourself which of your existing tools can help you?
Once I started really understanding GTD and became very proficient at it, I then started testing various tools to see if I could become even more productive. I tried several dozen tools and eventually came back to my original basic toolset of Evernote and my iPhone/iPad.
Remember that the most important element of GTD is keeping things simple. Don't try to over-complicate a methodology that tries to simplify your life.