This has been discussed on the planner-dev mailing list.  It's the point number
9 in this email
http://lists.imendio.com/pipermail/planner-dev/2004-September/001073.html

If you enter a task in the Gantt Chart where its name is longer than the current
Name column, it automatically resizes it.  Shouldn't it be the opposite?  I
don't want to be changing every time I add a new task the separator between the
table and the graph to see the Work column.

Richard Hult answer to the email:

I'm pretty much waiting for GTK+ 2.6 since it's trivial to fix with that 
version. We could add some kind of workaround for now though, that 
ellipsizes the text if it's too wide. It's probably a good idea.