GUISCREEN   GuiFile  FRONT|BACK

 This will bring the screen that the GuiFile is open on, to the 
 FRONT or move it to the BACK.

 No action will be taken if the GUI is not open.

 You can also give the ScreenName directly, by putting a # char
 in front :

   GuiScreen #Workbench Front

 will bring the workbench to the front.
 Note that screen names are case-sensitive.