Ok awesome news - it worked by using the zip file approach you linked to. Downloaded the ganttproject zip, set the export variable (had to do a search for liberica on my root to find out where it gets installed, no idea how to find out any other way) and ran the ganttproject from the zip folder, all from the command line. Open file dialogue works!
/home/domarius/.ganttproject.d/logs/.ganttproject-20210322125434.log
GanttProject 3.0.3004
Settings file:
location: /home/domarius/.ganttproject
size:4478
is readable: true
java.class.path: :/home/domarius/Downloads/ganttproject-3.0.3000/eclipsito.jar
java.home: /usr/lib/jvm/liberica-jre-11-full
java.ext.dirs: null
java.io.tmpdir: /tmp
java.runtime.version: 11.0.10+9-LTS
java.vendor: BellSoft
java.vm.name: OpenJDK 64-Bit Server VM
java.vm.vendor: BellSoft
java.vm.version: 11.0.10+9-LTS
os.arch: amd64
os.name: Linux
os.version: 5.9.16-1-MANJARO
sun.java.command: com.bardsoftware.eclipsito.Launch --verbosity 1 --version-dirs /home/domarius/Downloads/ganttproject-3.0.3000/plugins:~/.ganttproject.d/updates --app net.sourceforge.ganttproject.GanttProject -log true -log_file /home/domarius/.ganttproject.d/logs/.ganttproject-20210322125434.log
user.country: AU
user.dir: /home/domarius/Downloads/ganttproject-3.0.3000
user.home: /home/domarius
user.language: en
user.timezone: Australia/Brisbane
InteropFactory: cannot load com.sun.javafx.embed.swing.newimpl.InteropFactoryN
Mar. 22, 2021 2:54:35 AM net.sourceforge.ganttproject.GPLogger log
INFO: Deleting old auto-save files
Mar. 22, 2021 2:54:35 AM biz.ganttproject.LoggerImpl debug
FINE: Creating main frame...
Mar. 22, 2021 2:54:35 AM biz.ganttproject.LoggerImpl debug
FINE: 1. loading look'n'feels
Mar. 22, 2021 2:54:36 AM biz.ganttproject.LoggerImpl debug
FINE: 2. loading options
Mar. 22, 2021 2:54:36 AM biz.ganttproject.LoggerImpl debug
FINE: 3. creating menus...
Mar. 22, 2021 2:54:36 AM biz.ganttproject.LoggerImpl debug
FINE: 4. creating views...
Mar. 22, 2021 2:54:36 AM biz.ganttproject.LoggerImpl debug
FINE: 5. calculating size and packing...
Mar. 22, 2021 2:54:36 AM biz.ganttproject.LoggerImpl debug
FINE: 6. changing language ...
Mar. 22, 2021 2:54:36 AM biz.ganttproject.LoggerImpl debug
FINE: 7. first attempt to restore bounds
Mar. 22, 2021 2:54:36 AM biz.ganttproject.LoggerImpl debug
FINE: 8. finalizing...
So what should we do from here? Is this something you can address from your end, or something I need to tweak on my end? Eg. set up a script file to run this local version of GanttProject.