Would you need to make a script to make a GUI fit the entire screen or could you just resize it?

No, resize it to (1, 0, 1, 0)

That will make the X scale and Y scale 1. The "Scale" is the scale of the screen. If the size X scale is at 1, then it will be the size of the screen. If it's at 0.5, then it will be half the size of the screen. If 2, then twice the size. Therefore, setting the scale to 1 for both X and Y sizes will cover the whole screen.

Note that this "screen" does NOT include the Top Bar. Thus, I recommend making the Top Bar not transparent if you want a solid screen. Otherwise, you will have to offset your Y position by -36 pixels and offset your Y size by 36 pixels. (36 pixels is the Y size of the Top Bar I believe)

Yes. Add a Frame, make the size (1,0,1,0) and put the children of the gui inside it.

You should resize it to (1,0,1.1,0) then change the position to (0,0,-0.1,0) Thats how I did it