When exactly this message shows up?

It happens for me after downloading and then trying to install the software.  
Screencapture attached

Just to be sure: you downloaded it from our Google code site, right? 
http://code.google.com/p/ganttproject/downloads/list

Would you mind testing an updated version?

I just got the same problem.
A workaround is to use the Linux version and start it via the included shell 
script in Terminal.
When trying to launch the application bundle of the Mac version, I get an error 
dialog as stated above. Also, the Console logs the following messages (using a 
german system):


30.07.12 19:37:27,081 CoreServicesUIAgent[2025]: Error SecAssessmentCreate: Der 
Vorgang konnte nicht abgeschlossen werden. (OSStatus-Fehler -67061.)
30.07.12 19:37:28,780 com.apple.launchd.peruser.502[158]: 
([0x0-0x1c81c8].net.sourceforge.ganttproject[2023]) Exited: Killed: 9

So, any volunteer to test an updated package?

No problem, I'd test an update.

So, here is an update: http://dl.dropbox.com/u/5454466/GanttProject%202.5.4.zip
It is just a zipped GanttProject.app, not a dmg file. You should be able to 
unzip it and just double-click. Let me know if it runs fine or not. 

Thanks for volunteering!

I just tested it, the problem persisted.

31.07.12 14:02:19,482 CoreServicesUIAgent[3674]: Error SecAssessmentCreate: Der 
Vorgang konnte nicht abgeschlossen werden. (OSStatus-Fehler -67061.)
31.07.12 14:02:27,073 com.apple.launchd.peruser.502[158]: 
([0x0-0x30e30e].net.sourceforge.ganttproject[3679]) Exited: Killed: 9

I just found out that the problem is related to Mountain Lion's new GateKeeper 
feature. Normally, GateKeeper should just say the App is unsigned and starting 
it via context menu should offer the option to open it nonetheless. Instead we 
get a totally irritating message and no option to open the app while GateKeeper 
is active.
BUT: If you switch off GateKeeper in System Preferences, GanttProject (the 
official version as well as the update above) starts fine. Also, the system 
remembers the App was launched: re-enabling GateKeeper after the successful 
execution of the app is safe, it may be launched again regardless of 
GateKeeper's state.

Oh, thanks for this investigation. At least I understand now that I need to 
sign GanttProject.app. Let's do another try a little bit later

Apparently, Apple decided to earn some money by forcing developers to get 
Developer ID for the sole purpose of signing apps. So it will take a little bit 
longer.

Even open source/ non-profit developers need to pay to get an ID??

Yet, it should also be possible to execute non-signed apps with GateKeeper 
enabled:
Normally, the user should be able to start the app initially by right-clicking 
it, choosing "open" from the context menu, confirm that he wants to open an 
unsigned app and in case of a non-admin account enter admin credentials in 
order to save the permanent clearance for the app.

The problem here is, that this process does not work correctly, and I've got no 
idea why. Maybe it's a bug in GateKeeper.

Nonetheless a signed app would, of course, be preferable.

In the meantime, the workaround of switching GateKeeper off when launching 
GattProject for the first time (and turning it back on afterwards) works fine.

@maarten: I didn't see any exceptions, so I believe, yes. Even sysadmins who 
deploy their one-off scripts.

@silent: we're not going to make so much troubles with switching Gate Keeper on 
and off to our users.

Okay, it took just 5 days to get the certificates. I've signed 
GanttProject.app. Can you please test it again?
It is still here: http://dl.dropbox.com/u/5454466/GanttProject%202.5.4.zip

Sorry to inform you that the problem still persists. Only the error code in the 
log message changed to a different value:
08.08.12 09:12:43,695 CoreServicesUIAgent[619]: Error SecAssessmentCreate: Der 
Vorgang konnte nicht abgeschlossen werden. (OSStatus-Fehler -2147409622.)
08.08.12 09:12:46,835 com.apple.launchd.peruser.502[158]: 
([0x0-0x6b06b].net.sourceforge.ganttproject[622]) Exited: Killed: 9

The problem is definitely related to Gatekeeper, since the app works fine if 
Gatekeeper is turned off during the first launch and regardless of Gatekeeper's 
state once the quarantine flag got removed. It also works if I remove the 
quarantine flag manually after uncompressing the downloaded zip:
xattr -d com.apple.quarantine GanttProject\ 2.5.4.app

Yet the real problem obviously wasn't the missing signature in the first place 
(which seems logical in so far as an unsigned app should behave different and 
allow for launching via context menu without switching Gatekeeper off).

I just found a different Project on Google Code which appears to have solved 
the same issue:
http://code.google.com/p/processing/issues/detail?id=1134

It seems that I've managed to get through this. Now I have Mountain Lion 
running in a virtual machine on Snow Leopard host, and after some fight with 
certificates, I have a properly signed GanttProject.app. At least my virtual 
Mountain Lion doesn't suggest me to move it to the trash anymore. I'll publish 
Praha 2.5.5 in the next few days, and will appreciate if you test the distro 
before publishing.

Hey guys, can you test this package: 
http://dl.dropbox.com/u/5454466/ganttproject-2.5.5-r1253.dmg ?
It works fine on my virtual Mountain Lion, but I want to be sure that it works 
not only for me.

Works for me too. Good job!

Works fine also for me. Thanks, great job.

I downloaded ganttproject-2.5.5-r1256.dmg for my Mac running OSX 10.5.8 and it 
wouldn't run. I then downloaded JRE 1.6 and installed that, and it still won't 
run. When I try to run it, the app name appears in the menu bar for about a 
second and then disapperas, and nothing happens. Help please?

@24: Is it Intel Mac or Power PC? And can you please start another discussion 
thread, since it is clearly not related to Gatekeep?