Sometimes I accidentally sort by date and it messes up all my items that I had in a certain order. I can use the undo function if I catch this but if I don’t I have to manual resort all the items. Any way to undo this feature? or is there a workaround?
You need to click the column header to trigger sorting on. When you click, you see the tasks reordered and see the arrow which indicates sorting close to the mouse pointer. Undo works fine and reverts tasks reordering.
All that said, I agree that one can accidentally click the header and miss the changes. We’ll think what we can do with that.
Just in case if you’re facing the issue right now and need to get back the order of dozens of tasks, you may try using Help->Recover. It scans auto-save files in reverse chronological order (least recent goes first) and asks if you want to read the file into GanttProject. Chances are that you will find an auto-save with the project data right before you sorted the tasks
A potential solution here - and a nice feature to have anyway - is to enable sorting on Outline Number. I added the column to my default view and assumed that it would be possible to sort on this column. Unfortunately not 
Scrap my previous suggestion… just realised that the outline number automatically changes as tasks are re-ordered. Doh.
Would enabling sorting by ID fix this issue? In any case, I’m having the same problem.
The same workaround for restoring the order: use Undo if you didn’t close GanttProject or Help>Recover if you did.
ID are pretty meaningless and change pretty often when task structure changes.
I have the same problem, there is no way to keep and define an order of tasks that is not date based.
What is the current intended function of the outline column? Since it changes with every sort and has no dependence on any functional task attribute you can’t use it as a reference and it seems to have little, if any, practical usage.
It seems like the outline is what should be establishing the intended reading order of tasks and thus should not be changing when the tasks are sorted. It should only change when the tasks themselves are re-ordered. I can see where this might get a little tricky when the task order appearance changes from a sorts on other attributes. However, I think it’s manageable to have order maintained by Outline even when the tasks are not in an outline sorted order and for that order to be updated by drag and dropping tasks of different outlines.
Thus, I think a potential fix for this issue is to adjust how the outline is established and maintained and make it a real attribute which establishes task intended reading order. If implemented as such, then users can re-establish their intended reading order by sorting by outline.
Outline number is the same as in MS Project, but yes sorting changes task order defined by user. It would have been better if it didn’t, but we can’t do it with the current implementation of GanttProject internal structures, sorry.
Can you disable Sorting …can you add a option to disable sorting? That would solve the problem of unintended sorting.
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Same problem here, with GP v3.005. 
+1 To have a “Enable column sorting” feature on GUI.