Посоветуйте софт под Линукс для составления проектного плана

наверное ProjectLibre будет самый близкий к MS Project. Как Calc к Excel.

«Пойдет даже полный клон Microsoft Project» --- полного клона ты как раз не найдешь, все что будет похожее на MSP --- все хуже по возможностям.

В качестве простого средства, возможно, подойдет уже упомянутый здесь ProjectLibre  — но я его не использовал для практических задач, так что поищи другие отзывы.

Нашел негативный отзыв про ProjectLibre:

Has potential, but lacks any real support or even decent documentation. The «help» button takes you to a defunct website. The «community» support is months or years behind and often gives overly simplified answers to questions to make it look like they are doing something. Setting in this software have issues sticking. You may have to set a setting 5 times before it sticks. Tons of bugs. When changing a resources calendar, the software may recalculate a task to be a million days to complete. When copying a resource, if that resource is assigned anywhere, the copy will also be assigned. You cannot delete the copy's assignment without closing the application and reopening, then delete the resource assignment, then restart the software to see the change. Assigning calendars to a resource seems to work, but does not show correctly in the interface. The software often confuses hours with dollars so when a task costs 100,000 dollars, it is automatically scheduled to take 100,000 days to complete. It is clear that this software is being abandoned since these bugs have been present in the software for several months with no information on correcting them. Too bad, it has a lot of potential.