GTD is a productivity method created by David Allen and popularized in his best-selling book Getting Things Done. In Tiago's first online course, Get Stuff Done Like a Boss, he taught students how to implement this method, and it’s been a big influence on Tiago and his work. (This course is no longer offered).
Building a Second Brain builds on and extends the GTD philosophy to encompass not just tasks but all the other kinds of digital information you have to keep track of: the notes, files, bookmarks, photos, quotes, and research you use to do your work. It follows the same principle – keep everything in a trusted place outside your head – but applied to a much broader range of materials.
Here’s a more comprehensive article that explains how the two methods are related: Getting Things Done + Personal Knowledge Management.