What are some alternatives?
When comparing Calligra Plan and GanttProject, you can also consider the following products
Taskjuggler - TaskJuggler is a modern and powerful, Free and Open Source Software project management tool.
Wrike - Wrike is a flexible, scalable, and easy-to-use collaborative work management software that helps high-performance teams organize and accomplish their work. Try it now.
FoxPlan - Web-based project collaboration tool
Asana - Asana project management is an effort to re-imagine how we work together, through modern productivity software. Fast and versatile, Asana helps individuals and groups get more done.
GNOME Planner - Planner is the GNOME project management tool licensed under the GPLv2.
Basecamp - A simple and elegant project management system.