Unofficial solve for this issue below, run at your own peril (very dramatic music plays 
)
I couldn’t find an example of this issue in the posts and I think it was originally caused by installing 3.3.3316 in parallel to 3.2.xx then uninstalling 3.2.xx. IMO that’s a rare edge case and not worth anyone wasting time trying to officially solve.
What I did might, or might not, fix it for you too, and it’s not a true “fix” for the root cause but it did eliminate the issue for me so, sharing…
Symptom: Installation from GP version 3.3.3316, auto update detects 3.3.3320 and 3.3.3322 updates are outstanding, press update, updater displays “installing 3.3.3320” and reaches higher than 93% (maybe gets to 100% but too fast to see) and then stops with Update Failed error:
org.bouncycastle.openpgp.PGPRuntimeOperationException: unable to verify signature: Bad signature length: got 384 but was expecting 256
The “fix” (that eventually worked):
- Uninstall GanttProject
- ensure c:\program files\ganttproject 3.3\ is gone
- delete any ganttproject-update<*>zip files from c:\users\appdata\local\temp
- delete c:\users\.ganttproject.d ← I think this was the final fix step cause the problem persisted over several re-installs until this was deleted and there were old 3.2.xx updates in there still
- download the latest installer, in my case this was the same as before 3.3.3316 ← I did this at the same time as step 4, no doing that isn’t perfect testing practice, and the downloads appeared to be identical, but no I didn’t crc check them either, meh it’s not an official fix so
- Let GanttProject start after install and it immediately runs the updater. Let it update without opening any files etc.
- Updates 3.3.3320 and 3.3.3322 were detected and both installed without error.
- GanttProject scheduling time!
