I have also discovered many posts complaining that KGpg does not appear to run. KGpg is a KDE application and it turns out that after the first execution, the default behaviour in any subsequent invocation is to minimise to system tray.
System Tray in Ubuntu is different from that in KUbuntu and its subsequent default start up mode gives the illusion that the program fails to run. It is running OK except that it has been minimised to system tray.
Hence after configuring your KGpg for the first time, you will not see it any more in Ubuntu. Not even if you reinstall it.
This is because KGpg writes the settings to this file:
and uninstallation does not remove this file.
If you invoke KGpg in Ubuntu and does not see its window showing up, do this:
1) Use System Monitor to terminate the KGpg process, if it is running.
2) In the Terminal, type:
to edit the configuration file
3) In the [User Interface] section add the following line:
4) Save the file and restart Kgpg.
During the first installation of KGpg, you can change the default not to use system tray icon in Settings > Config KGpg > Misc > Applet & Menus. If you have forgotten to do it in that phrase, you then need to edit the configuration settings manually as shown above.