Downloaded and Installed Table 'OS/2' does not exist in /System/Library/Fonts/AppleGothic.ttf
shows as detail of "Errors Happened"
Installation of ganttproject-2.0.10.dmg
This is not related to installation. GP (one of the libraries actually) can't read one of the font files on your system, but that's ok, provided export works.
I see the same error message everytime I start GanttProject on my Mac.
It's probably not related to installation - but it's disturbing nevertheless.
If this is not an important error message it's better to suppress it.
Exporting a png image worked.