There's a little problem with the way the question is worded... it presupposes an answer that belongs to the same category of solutions as GTD.
GTD is a collection of practices that work well when used in concert with each other. Presumably, it's the system David Allen (the author) uses.
However, the reality is that many people don't use all his practices. Instead, they make up their own. Whether they are actually "doing GTD" or not is an interesting question, but most people don't care. It's probably not an exaggeration to say that billions of people are using elements of GTD without having any awareness of what it is.
Does it matter?
Of course not. There's something basic happening in the life of every human being that's much bigger than GTD.
In my book I share research from a wide number of scientists showing that human beings develop their own time management system... a collection of habits, practices and rituals that vary widely from each other. Adult learning principles also tell us that the best way to help someone improve a system they have already in place is not to offer them a completely new system.set of practices... such as GTD. This approach - pedagogy - happens to work with kids but "andragogy" works with adults.
Here's the approach I recommend - ETaPS - which is informed by andragogy.
E- Evaluate your current system (and its component behaviors) against best practices (including those in GTD).
Ta - Set new Targets for specific behaviors
P - create a Plan for changing behaviors and learning new habits in small steps
S - set up Support systems to make sure the changes stick
This approach isn't inimical to the ideas in GTD, many of which are world-class. However, it's asking a very different core question that leads to a better outcome. Someone who reads GTD is left trying to adapt a slew of new habits, practices and behaviors. Many fail for a number of reasons. The problem doesn't lie in GTD... the fault is in their approach to self-learning (heutagogy). They ask themselves "How can I copy David Allen's system?"
There's greater success to be gained using ETaPS, because it matches the reality people face more closely. Instead, they ask themselves "What am I currently doing, and how should I improve it?"
Having said that, very few people expect GTD to be the final answer, although it has many fine answers. There will be enhancements and improvements that go beyond the fixed practices the book suggests. In fact, David Allen's thinking has evolved since 2001 when he wrote GTD. His new thinking hasn't made it into a book, but if you check out his forums you can see where he has switched from saying that a calendar should only be used for appointments i.e. someone's hard landscape.
(I'd say this a timely switch, because AI-based apps like Timeful and SkedPal remove the barriers he listed back in 2001 to putting every task in your calendar.)
This evolution is only natural. The best way to think about alternatives to GTD is not to look for replacement systems that offer more/better/different fixed behaviors. That's more of the same.
Instead, it's better to see how you have evolved your own time management system to this point, and how you are likely to keep that evolution going in the future. In this context, you can be open to suggestions from every source, because you are guiding your own behavior change one step at a time.
I hope this helps,