Open source alternatives for MS Project

Few open source alternatives available to read MS Project files.

1. GanttProject
Its Eclipse based software tool available in GNU GPL license. It can import any MS Project file and export changes back to MS Project format.

Currently, this is my choice for free/ open source alternative for MS Project.

2. another option is Planner
This software is also available in GNU GPL license terms. But this cann’t important any MS Project format, but supports MS Project XML format for viewing.