Comment # 4 on bug 920510 from Dominique Leuenberger

pkexec IS the way to launch it graphically.. and it is what is in the .desktop
file to start it.

gksu/gnomesu... all those are not the way to do things in 2015 :) pkexec is
policykit controlled and is exactly the right thing.

of course I have no idea what 'the small error window' is that pops up and what
it says exactly... but it MIGHT be the dialog asking for the root password..
then that would be pretty normal (gnomesu/gksu would do the same: they'd ask
for the root password)