This post has haunted me because its the second result on google, so I think its necessary that I give a complete answer.
In the Gui object, make sure that “IgnoreGuiInset” is enabled, then inside the frame you want to cover the screen, set its size to “1,0,1,0.” That will cover the entire screen and ignore the top bar area 
Also check if the position or anchorpoint values of the frame aren’t offsetting anything at all