GTasks vs Todo.txt

Unlimited subtasks or sub sub tasks, as deep as you need, excellent for project work.

Seamlessly syncs tasks.

Todo.txt can be edited in plain text and then displayed with neat styles.

Because todo.txt is just a todo standard with various compatible apps to edit it, you can update it from any device as long as it can edit text and you can access your todo.txt file

It's convenient to be able to access your todo list quickly via your programming environment so you don't have to switch contexts, and to have a separate todo.txt file per project.

Todo.txt lists can be edited in virtually every text editor ever made and accessed on devices that support Dropbox. The user community has also created a number of apps for dozens of platforms.

You can edit your lists using the command line (e.g., Terminal) via a supplied bash script.

You have full access to the underlying programming rules, and therefore total control over your task lists.

Has been extended to include most things people want, including a multiple of interfaces and app for all platforms.

Todo.txt automatically and seamlessly syncs through Dropbox.

The community has developed a wide range of apps that interface with the basic file structure. These apps vary in how much they accomplish, how much hand-holding they do, their level of glitz, etc. They range from plugins for an editing app like vim to stand-alone, platform-specific GUI's. So YMMV depending on what you use. (todopy is particularly nice, a Python program that has a "console" mode which provides a keyboard-driven GUI.)

Does one thing: to do lists

It runs on MacOS, Android, and Python (in the Mac's Unix CLI) -- all for free.

At a bare minimum you just need to be able to edit text, but there are many apps and advanced interfaces you can install on many devices to supplement your to-do list editing.