Press Alt + F2 then write : gksu-properties
and open it.
The first option should be "sudo"
Authentication mode: sudo
and change the second option from "enable" to "force enable".
Grab mode : force enable
Then retry to open an application with gksu
You can also try the gksudo
instead of gksu
Maybe gksudo
works better.
Also you can try to create a new launcher
Open a terminal and issue the following commands
sudo apt-get install --no-install-recommends gnome-panel
gnome-desktop-item-edit ~/.local/share/applications/ --create-new
At the opened window complete the appropriate sections
Name: wireshark-root
Command: gksudo wireshark
Comment: anything you want
Find the new launcher and try to open it.
Alternatively you can try to replace the Command: gksudo wireshark
with Command: gksudo wireshark &
maybe the ampersand &
helps here.