GTD is an approach that is not tool-specific. So while it’s important to land on gear (paper or digital) that will stand up to the complexity of your work and personal life, it’s more important that it clearly serves the purpose of reflecting the reminders and information in the most appropriate way for you. The tool won’t decide what something means—you have to do that, and the GTD process is the key.
If anyone is telling you a specific piece of software is required for GTD–good chance they don’t understand GTD.
We do have a few key tools that we personally use and recommend that have gone through David Allen’s extensive vetting process. You can find a link to those in our online store. If you don’t see your software listed here, it means we do not have a recommendation at this time, but a search on GTD Times and our public Forums should give you quite a few helpful suggestions about what other GTD implementers are using.