How would you make a Gui fit the whole screen?

GUIs have two dimensions, height and width (or x and y), but they are four Size arguments. They have xScale,xOffset,yScale,yOffset. Offset is the number of pixels the GUI takes up; it will not vary due to screen size. Scale is a percentage. Although it's a percentage, it gets converted into a decimal, so it's a number between 0 and 1. 1 is 100%, 0.5 is 50%, etc. Since scale is a percentage, setting both scales to 1 will make it cover the entire screen.