Prozilla works in textmode were as prozGUI is graphical.I use prozilla,and therefore i dont know about prozGUI.
Incase,if you are asking how to start it,just search for where it is installed like,
and then run with full path name.also,try just type'proz' and then press TAB.or just locate prozgui like 'locate prozgui' after an 'updatedb'