If you want to have a simple yet effective todo manager, this application is for you. It is an open source application for managing your personal tasks and todo list. One of useful features are having composite tasks which you can set this by contexts or projects. It should able to convert for using GTD method. Here is its introduction:
Task Coach is a simple open source todo manager to manage personal tasks and todo lists. It grew out of my frustration that well-known task managers, such as those provided with Outlook or Lotus Notes, do not provide facilities for composite tasks. Often, tasks and other things todo consist of several activities. Task Coach is designed to deal with composite tasks.
Task Coach – Your friendly task manager
View more information: https://www.lifehack.org/articles/lifehack/download-of-the-day-task-coach.html