Was really thrown by all the references to "GTD". It seems like a heavily commercialized productivity workflow?
My entire productivity system revolves around Vikunja[1]. It's essentially just a todo list application, but it's super flexible (e.g. view as list, kanban, gantt, etc), supports having many lists (I keep one per different idea/topic), and so many other features. You can also self-host it for free, which has been a completely painless process for me personally, and gives you the standard peace of mind from being in control of your personal data.
Getting Things Done is a book that describes a system for... Well you get it. There doesn't have to be anything commercial about it, you can find synopses online for free and the system is simple so those are probably nearly as good as reading the book.
I read it about ten years ago and still use some parts of it, it's a good system and probably becomes more and more useful the busier you are. I'm sure it could be used alongside the app you shared. Which looks nice btw, I've been looking for something self hosted to use for a small team. I'll check it out.
There are commercial training courses, which most people don't take. The recommended techniques are based on pen and paper - so no real potential for commercialisation there. Of course most people born after 1960 will use some form of computer assistance, but that's not the GTD people selling product.
My entire productivity system revolves around Vikunja[1]. It's essentially just a todo list application, but it's super flexible (e.g. view as list, kanban, gantt, etc), supports having many lists (I keep one per different idea/topic), and so many other features. You can also self-host it for free, which has been a completely painless process for me personally, and gives you the standard peace of mind from being in control of your personal data.
Highly suggest checking it out!
[1] https://vikunja.io/